mlxsw: Make EtherType configurable when pushing VLAN at ingress

Currently, when pushing a PVID at ingress, mlxsw always uses 802.1q
EtherType.

Make this EtherType configurable by extending mlxsw_sp_port_pvid_set()
with an EtherType argument.

This is a preparation for QinQ support, that needs to push a PVID with
802.1ad EtherType.

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>