USB: u_ether: Fix race between gether_disconnect and rx_submit

Race is happening when both rx_submit() and gether_disconnect()
executes in parallel. When rndis is disabled, in
gether_disconnect() dev port_usb is set to NULL. On other side,
rx_sumit() executes in parallel and dev port_usb is accesed
leading to NULL pointer dereference.

Fix the issue by extending the spin_lock protection for the
dev port_usb in rx_submit().

Change-Id: Ie788cde9dac56230046503be68ad3ab8a9839eec
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>