usb: gadget: s3c2410_udc: Do not use integer for NULL

'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: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>