usb: u_ether: Add null pointer check for sk_buff in eth_start_xmit

There is a possibility of null pointer dereference for sk_buff in
the function eth_start_xmit. Fix this by removing sk_buff from if
check at the start of the function eth_start_xmit so that if sk_buff
is null dev_kfree_skb_any will return without proceeding further.

Change-Id: I105e42c59aaa83c8d74cc5a9e894ae4fc87c3c9f
Signed-off-by: Rohith Kollalsi <rkollals@codeaurora.org>