mlxsw: spectrum_switchdev: Create common functions for VLAN-aware bridge

The code in mlxsw_sp_bridge_8021q_port_{join, leave}() can be used also
for 802.1ad bridge.

Move the code to functions called
mlxsw_sp_bridge_vlan_aware_port_{join, leave}() and call them from
mlxsw_sp_bridge_8021q_port_{join, leave}() respectively to enable code
reuse.

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>