igc: Check VLAN EtherType mask

Currently the driver accepts VLAN EtherType steering rules regardless of
the configured mask. And things might fail silently or with confusing error
messages to the user. The VLAN EtherType can only be matched by full
mask. Therefore, add a check for that.

For instance the following rule is invalid, but the driver accepts it and
ignores the user specified mask:
|root@host:~# ethtool -N enp3s0 flow-type ether vlan-etype 0x8100 \
|             m 0x00ff action 0
|Added rule with ID 63
|root@host:~# ethtool --show-ntuple enp3s0
|4 RX rings available
|Total 1 rules
|
|Filter: 63
|        Flow Type: Raw Ethernet
|        Src MAC addr: 00:00:00:00:00:00 mask: FF:FF:FF:FF:FF:FF
|        Dest MAC addr: 00:00:00:00:00:00 mask: FF:FF:FF:FF:FF:FF
|        Ethertype: 0x0 mask: 0xFFFF
|        VLAN EtherType: 0x8100 mask: 0x0
|        VLAN: 0x0 mask: 0xffff
|        User-defined: 0x0 mask: 0xffffffffffffffff
|        Action: Direct to queue 0

After:
|root@host:~# ethtool -N enp3s0 flow-type ether vlan-etype 0x8100 \
|             m 0x00ff action 0
|rmgr: Cannot insert RX class rule: Operation not supported

Fixes: 2b477d05 ("igc: Integrate flex filter into ethtool ops")
Suggested-by: Suman Ghosh <sumang@marvell.com>
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>