USB: gadget: u_ether: Fix NULL pointer dereference issue

When cable is disconnected during RNDIS data transfers, there
is a chance that eth_start_xmit() and disable callback race
each other and results in NULL pointer deference. Hence
fix it by adding is_fixed, fixed_in_len variables in
eth_start_xmit function and assign the values if port_usb
pointer is not NULL.

Change-Id: If4b4452fc7f110efd51aaedd55a1f3223d9957b6
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>
Signed-off-by: ChandanaKishori Chiluveru <cchilu@codeaurora.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>