usb: gadget: Introduce usb_gadget_giveback_request()

All USB peripheral controller drivers call completion routines directly.
This patch adds usb_gadget_giveback_request() which will be used instead
of direct invocation in the next patch. The goal here is to have a place
where common functionality can be added.

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>