usb: gadget: gr_udc: Use GFP_ATOMIC when allocating under held spinlock

As gr_ep_init must be called with dev->lock held, GFP_KERNEL must not be used.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>