usb: gadget: fix unused-but-set-variale warnings

Those are enabled with W=1 make option.

The patch leaves of some type-limits warnings which are caused by
generic macros used in a way where they produce always-false
conditions.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>