s390/netiucv: improve endianness handling

Replace ntohs with endianness conversion for the SKB protocol assignment
to avoid an endianness warning reported by sparse. No functional change.

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>