usb: gadget: goku_udc: use NULL instead of 0

'req' is a pointer; thus, use NULL instead of 0
to fix the following sparse warning:

drivers/usb/gadget/goku_udc.c:775:13: warning: Using plain integer as NULL pointer

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>