USB: qcserial: Add support for Quectel EC20 Mini PCIe module

It seems like this device has same vendor and product IDs as G2K
devices, but it has different number of interfaces(4 vs 5) and also
different interface layout which makes it currently unusable:

	usbcore: registered new interface driver qcserial
	usbserial: USB Serial support registered for Qualcomm USB modem
	usb 2-1.2: unknown number of interfaces: 5

lsusb output:

	Bus 002 Device 003: ID 05c6:9215 Qualcomm, Inc. Acer Gobi 2000 Wireless
	Device Descriptor:
	  bLength                18
	  bDescriptorType         1
	  bcdUSB               2.00
	  bDeviceClass            0 (Defined at Interface level)
	  bDeviceSubClass         0
	  bDeviceProtocol         0
	  bMaxPacketSize0        64
	  idVendor           0x05c6 Qualcomm, Inc.
	  idProduct          0x9215 Acer Gobi 2000 Wireless Modem
	  bcdDevice            2.32
	  iManufacturer           1 Quectel
	  iProduct                2 Quectel LTE Module
	  iSerial                 0
	  bNumConfigurations      1
	  Configuration Descriptor:
	    bLength                 9
	    bDescriptorType         2
	    wTotalLength          209
	    bNumInterfaces          5
	    bConfigurationValue     1
	    iConfiguration          0
	    bmAttributes         0xa0
	      (Bus Powered)
	      Remote Wakeup
	    MaxPower              500mA

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Cc: stable <stable@vger.kernel.org>
[johan: rename define and add comment ]
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/qcserial.c

@@ -22,6 +22,8 @@
 #define DRIVER_AUTHOR "Qualcomm Inc"
 #define DRIVER_DESC "Qualcomm USB Serial driver"
 
+#define QUECTEL_EC20_PID	0x9215
+
 /* standard device layouts supported by this driver */
 enum qcserial_layouts {
 	QCSERIAL_G2K = 0,	/* Gobi 2000 */
@@ -171,6 +173,38 @@ static const struct usb_device_id id_table[] = {
 };
 MODULE_DEVICE_TABLE(usb, id_table);
 
+static int handle_quectel_ec20(struct device *dev, int ifnum)
+{
+	int altsetting = 0;
+
+	/*
+	 * Quectel EC20 Mini PCIe LTE module layout:
+	 * 0: DM/DIAG (use libqcdm from ModemManager for communication)
+	 * 1: NMEA
+	 * 2: AT-capable modem port
+	 * 3: Modem interface
+	 * 4: NDIS
+	 */
+	switch (ifnum) {
+	case 0:
+		dev_dbg(dev, "Quectel EC20 DM/DIAG interface found\n");
+		break;
+	case 1:
+		dev_dbg(dev, "Quectel EC20 NMEA GPS interface found\n");
+		break;
+	case 2:
+	case 3:
+		dev_dbg(dev, "Quectel EC20 Modem port found\n");
+		break;
+	case 4:
+		/* Don't claim the QMI/net interface */
+		altsetting = -1;
+		break;
+	}
+
+	return altsetting;
+}
+
 static int qcprobe(struct usb_serial *serial, const struct usb_device_id *id)
 {
 	struct usb_host_interface *intf = serial->interface->cur_altsetting;
@@ -240,6 +274,12 @@ static int qcprobe(struct usb_serial *serial, const struct usb_device_id *id)
 			altsetting = -1;
 		break;
 	case QCSERIAL_G2K:
+		/* handle non-standard layouts */
+		if (nintf == 5 && id->idProduct == QUECTEL_EC20_PID) {
+			altsetting = handle_quectel_ec20(dev, ifnum);
+			goto done;
+		}
+
 		/*
 		 * Gobi 2K+ USB layout:
 		 * 0: QMI/net