USB: serial: sierra: fix bogus alternate-setting assumption

Interface numbers do not change when enabling alternate settings as
comment and code in this driver suggested.

Remove the confusing comment and redundant retrieval of the interface
number in probe, while simplifying and renaming the interface-number
helper.

Fixes: 4db2299da213 ("sierra: driver interface blacklisting")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/sierra.c

@@ -137,24 +137,9 @@ static int is_himemory(const u8 ifnum,
 	return 0;
 }
 
-static int sierra_calc_interface(struct usb_serial *serial)
+static u8 sierra_interface_num(struct usb_serial *serial)
 {
-	int interface;
-	struct usb_interface *p_interface;
-	struct usb_host_interface *p_host_interface;
-
-	/* Get the interface structure pointer from the serial struct */
-	p_interface = serial->interface;
-
-	/* Get a pointer to the host interface structure */
-	p_host_interface = p_interface->cur_altsetting;
-
-	/* read the interface descriptor for this active altsetting
-	 * to find out the interface number we are on
-	*/
-	interface = p_host_interface->desc.bInterfaceNumber;
-
-	return interface;
+	return serial->interface->cur_altsetting->desc.bInterfaceNumber;
 }
 
 static int sierra_probe(struct usb_serial *serial,
@@ -165,7 +150,7 @@ static int sierra_probe(struct usb_serial *serial,
 	u8 ifnum;
 
 	udev = serial->dev;
-	ifnum = sierra_calc_interface(serial);
+	ifnum = sierra_interface_num(serial);
 
 	/*
 	 * If this interface supports more than 1 alternate
@@ -178,9 +163,6 @@ static int sierra_probe(struct usb_serial *serial,
 		usb_set_interface(udev, ifnum, 1);
 	}
 
-	/* ifnum could have changed - by calling usb_set_interface */
-	ifnum = sierra_calc_interface(serial);
-
 	if (is_blacklisted(ifnum,
 				(struct sierra_iface_info *)id->driver_info)) {
 		dev_dbg(&serial->dev->dev,
@@ -342,7 +324,7 @@ static int sierra_send_setup(struct usb_serial_port *port)
 
 	/* If composite device then properly report interface */
 	if (serial->num_ports == 1) {
-		interface = sierra_calc_interface(serial);
+		interface = sierra_interface_num(serial);
 		/* Control message is sent only to interfaces with
 		 * interrupt_in endpoints
 		 */
@@ -916,7 +898,7 @@ static int sierra_port_probe(struct usb_serial_port *port)
 	/* Determine actual memory requirements */
 	if (serial->num_ports == 1) {
 		/* Get interface number for composite device */
-		ifnum = sierra_calc_interface(serial);
+		ifnum = sierra_interface_num(serial);
 		himemoryp = &typeB_interface_list;
 	} else {
 		/* This is really the usb-serial port number of the interface