net: enetc: fix incorrect TPID when receiving 802.1ad tagged packets

When the enetc ports have rx-vlan-offload enabled, they report a TPID of
ETH_P_8021Q regardless of what was actually in the packet. When
rx-vlan-offload is disabled, packets have the proper TPID. Fix this
inconsistency by finishing the TODO left in the code.

Fixes: d4fd0404 ("enetc: Introduce basic PF and VF ENETC ethernet drivers")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>