s390/qeth: improve endianness handling

Avoid endianness warnings reported by sparse by (1) using endianness
conversions for assigning and using network packet fields, and (2)
removing unnecessary endianness conversions from qeth_l3_rebuild_skb. No
functional changes.

Signed-off-by: Hans Wippel <hwippel@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>