ethtool: allow flow-type ether without IP protocol field

Set IP protocol mask only when IP protocol field is set.
This will allow flow-type ether with vlan rule which don't have
protocol field to apply.

ethtool -N ens5f4 flow-type ether proto 0x8100 vlan 0x600\
m 0x1FFF action 3 loc 16

Signed-off-by: Vishal Kulkarni <vishal@chelsio.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>