mlxsw: spectrum: Add mlxsw_sp_port_egress_ethtype_set()

A subsequent patch will cause decapsulated packets to have their EtherType
determined by the egress port. Add mlxsw_sp_port_egress_ethtype_set() which
will be called when a port joins an 802.1ad bridge, so that it will set an
802.1ad EtherType on decapsulated packets transmitted through it, instead
of the default 802.1q EtherType.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>