mlxsw: reg: Add et_vlan field to SPVID register

et_vlan field is used to configure which EtherType is used when VLAN is
pushed at ingress (for untagged packets or for QinQ push mode).

It will be used to configure tagging with ether_type1 (i.e., 0x88A8) for
QinQ mode.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>