usb/gadget: Remove set-but-not-used variables

Avoid that building with W=1 triggers compiler warnings about
set-but-not-used variables.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Andy Grover <agrover@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>