mhi: dev: netdev: Set the mac header to 0 for skbs

The alloc_skb call set the mac_header to ~0 which
can cause the skb data to point to an invalid location.
Fix this by setting the mac_header to 0 before submitting
it.

Change-Id: I79bdf3528c2b7961c6e1916936780f45680b1f9f
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>