net/iucv: improve endianness handling

Use proper endianness conversion for an skb protocol assignment. Given
that IUCV is only available on big endian systems (s390), this simply
avoids an endianness warning reported by sparse.

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