usb: gadget: udc: gr_udc: fix memleak on error handling path in gr_ep_init()

gr_ep_init() does not assign the allocated request anywhere if allocation
of memory for the buffer fails. This is a memory leak fixed by the given
patch.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Evgeny Novikov <novikov@ispras.ru>
Signed-off-by: Felipe Balbi <balbi@kernel.org>