usb: gadget: net2280: Dont use 0 as NULL pointer

Fix sparse warning

Fixes: cb442ee1 (usb: gadget: udc: net2280: Re-enable dynamic debug messages)
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>