usb: gadget: u_audio: Fixed variable declaration coding style issue

Fixed a coding style issue, replacing unsigned with unsigned int.

Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Jonas Stenvall <jonas.stenvall.umea@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>