s390/ctcm: improve endianness handling

Use endianness conversions for SKB protocol assignments and usage to
avoid endianness warnings reported by sparse. 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>