usb: gadget: function: u_serial: Repair misdocumented function argument

Looks as though this has been an issue since the driver's
inception back in 2014.

Fixes the following W=1 kernel build warning(s):

 drivers/usb/gadget/function/u_serial.c:538: warning: Function parameter or member 'port' not described in 'gs_start_io'
 drivers/usb/gadget/function/u_serial.c:538: warning: Excess function parameter 'dev' description in 'gs_start_io'

Cc: Felipe Balbi <balbi@kernel.org>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Al Borchers <alborchers@steinerpoint.com>
Cc: Peter Berger <pberger@brimson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200703174148.2749969-27-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/u_serial.c

@@ -527,7 +527,7 @@ static int gs_alloc_requests(struct usb_ep *ep, struct list_head *head,
 
 /**
  * gs_start_io - start USB I/O streams
- * @dev: encapsulates endpoints to use
+ * @port: port to use
  * Context: holding port_lock; port_tty and port_usb are non-null
  *
  * We only start I/O when something is connected to both sides of