usb: gadget: u_ether: fix another dereference after null check

dev->port_usb is checked for null pointer previously, so dev->port_usb
might be null during no zlp check, fix it by adding null pointer check.

Acked-by: Jim Baxter <jim_baxter@mentor.com>
Signed-off-by: Harish Jenny K N <harish_kandiga@mentor.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>