usb: gadget: omap_udc: remove possiblity of NULL pointer de-reference

when allocating a request, it's better programming
practice to make sure we return NULL if allocation
failed.

This will ensure that, if struct usb_request isn't
the first member on our structure, we don't cheat
the gadget driver into thinking allocating worked
because pointer isn't 0.

Signed-off-by: Felipe Balbi <balbi@ti.com>