usb: gadget: Refactor request completion

Use the recently introduced usb_gadget_giveback_request() in favor of
direct invocation of the completion routine.

All places in drivers/usb/ matching "[-.]complete(" were replaced with a
call to usb_gadget_giveback_request(). This was compile-tested with all
ARM drivers enabled and runtime-tested for musb.

Signed-off-by: Michal Sojka <sojka@merica.cz>
Acked-by: Felipe Balbi <balbi@ti.com>
Tested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>