usb: gadget: NCM: Protect dev->port_usb using dev->lock

This commit incorporates findings from
https://lkml.org/lkml/2016/4/25/594



The function has been modified to make sure we hold
the dev lock when accessing the net device pointer.

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>