usb: gadget: gr_udc: unconditionally use GFP_ATOMIC in gr_queue_ext()

As far as gr_queue() is called with spinlock held,
we have to pass GFP_ATOMIC regardless of gfp argument.

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

Acked-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Felipe Balbi <balbi@ti.com>