usb: gadget: net2280: fix sparse warnings

fix the following sparse warnings:

drivers/usb/gadget/net2280.c:2063:13: warning: Using plain integer as NULL pointer
drivers/usb/gadget/net2280.c:2321:68: warning: Using plain integer as NULL pointer
drivers/usb/gadget/net2280.c:2349:68: warning: Using plain integer as NULL pointer
drivers/usb/gadget/net2280.c:2371:68: warning: Using plain integer as NULL pointer

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>