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

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.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>