s390/qeth: unionize next-hop field in qeth L3 header

The L3 packet descriptor's 'dest_addr' field is used for a different
purpose in RX descriptors. Clean up the hard-coded byte accesses and
try to be more self-documenting.

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