usb: gadget: cleanup on stack DECLARE_COMPLETIONs

fixups for incorrect use of DECLARE_COMPLETION. see also commit
6e9a4738 ("completions: lockdep annotate on stack completions")

patch is against 3.18.0 linux-next

This was only code reviewed and compile tested

Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Felipe Balbi <balbi@ti.com>