USB: u_ether: Check if port_usb is NULL before accessing

Rndis tx requests are queued to the controller, while disconnect
is in progress. There is a chance of getting the completion of
those requests after dev->port_usb is set to NULL. This leads to
NULL pointer dereference in tx_complete().

Hence, check if dev->port_usb is NULL before accessing it
in tx_complete() and free the current request accordingly.

Change-Id: I42a427f7e51ffe4f81768756d835d123778df260
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>