usb: gadget: u_ether: Fix NULL pointer dereference issue

dev->port_usb can be NULL if USB cable is disconnected when
net device is trying to transfer data which is causing NULL
pointer dereference. Hence add changes to eliminate need of
accessing port->port_usb to see multi_xfer_pkt value.

Change-Id: Ia28399b896ec1e069abeee8fe477826de6bb5942
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>