'req' being a pointer shouldn't be equated with 0.

Fixes the following compilation warning:
drivers/usb/gadget/s3c2410_udc.c:1299:13: warning:
Using plain integer as NULL pointer
Signed-off-by: NSachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: NFelipe Balbi <balbi@ti.com>