usb: gadget: udc: add null check before pointer dereference

Add null check before dereferencing dev->regs pointer inside
net2280_led_shutdown() function.

Addresses-Coverity-ID: 101783
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>