net: bcmgenet: use __be16 for htons(ETH_P_IP)

The 16-bit value that holds a short in network byte order should
be declared as a restricted big endian type to allow type checks
to succeed during assignment.

Fixes: 3e370952 ("net: bcmgenet: add support for ethtool rxnfc flows")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Doug Berger <opendmb@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>