USB: serial: oti6858: fix NULL-deref at open

Fix NULL-pointer dereference in open() should the device lack the
expected endpoints:

Unable to handle kernel NULL pointer dereference at virtual address 00000030
...
PC is at oti6858_open+0x30/0x1d0 [oti6858]

Note that a missing interrupt-in endpoint would have caused open() to
fail.

Fixes: 49cdee0ed0fc ("USB: oti6858 usb-serial driver (in Nokia CA-42
cable)")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/oti6858.c

@@ -134,6 +134,7 @@ static int oti6858_chars_in_buffer(struct tty_struct *tty);
 static int oti6858_tiocmget(struct tty_struct *tty);
 static int oti6858_tiocmset(struct tty_struct *tty,
 				unsigned int set, unsigned int clear);
+static int oti6858_attach(struct usb_serial *serial);
 static int oti6858_port_probe(struct usb_serial_port *port);
 static int oti6858_port_remove(struct usb_serial_port *port);
 
@@ -158,6 +159,7 @@ static struct usb_serial_driver oti6858_device = {
 	.write_bulk_callback =	oti6858_write_bulk_callback,
 	.write_room =		oti6858_write_room,
 	.chars_in_buffer =	oti6858_chars_in_buffer,
+	.attach =		oti6858_attach,
 	.port_probe =		oti6858_port_probe,
 	.port_remove =		oti6858_port_remove,
 };
@@ -324,6 +326,20 @@ static void send_data(struct work_struct *work)
 	usb_serial_port_softint(port);
 }
 
+static int oti6858_attach(struct usb_serial *serial)
+{
+	unsigned char num_ports = serial->num_ports;
+
+	if (serial->num_bulk_in < num_ports ||
+			serial->num_bulk_out < num_ports ||
+			serial->num_interrupt_in < num_ports) {
+		dev_err(&serial->interface->dev, "missing endpoints\n");
+		return -ENODEV;
+	}
+
+	return 0;
+}
+
 static int oti6858_port_probe(struct usb_serial_port *port)
 {
 	struct oti6858_private *priv;