s390/qeth: extract helper to determine L2 cast type

This de-duplicates the L2 and L3 cast-type code, and makes the L2 code
a bit more robust by removing the fragile assumption that skb->data
always points to the Ethernet Header. This would break in code paths
where we pushed the HW header onto the skb.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>