usb: gadget: u_ether: fix dereference after null check coverify warning

dev->port_usb is checked for null pointer at above code, so dev->port_usb
might be null, fix it by adding null pointer check.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>