usb: gadget: u_ether: Fix NULL pointer derefernce crash

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

CRs-Fixed: 452642
Change-Id: I60bd2302371238707e951c77912b155d75dc8570
Signed-off-by: Mayank Rana <mrana@codeaurora.org>