usb: gadget: u_ether: Add skb check in eth_start_xmit

'Commit 145c3d9be206 ("usb: u_ether: Prevent dropping multicast
packet in rmnet ip mode")' removed skb check from eth_start_xmit
function. While ndo_start_xmit are typically expected never to be
called with NULL SKBs, for NCM function this xmit function is
called with NULL SKB initially which then uses the 'wrap' routine
to create valid SKBs. But this null SKB is getting dereferenced
before 'wrap' routine could run on it when in ethernet mode.
Fix this by bringing in the skb check.

Change-Id: I9d40f70190eb91ecc848b71c4ce31a656c60a28d
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>