qlcnic: fix panic while using eth_hdr

o skb->mac_header is not set, so machine panics while using function eth_hdr.

Signed-off-by: Rajesh Borundia <rajesh.borundia@qlogic.com>
Signed-off-by: Amit Kumar Salecha <amit.salecha@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>