mlxsw: spectrum_switchdev: Create common function for joining VxLAN to VLAN-aware bridge

The code in mlxsw_sp_bridge_8021q_vxlan_join() can be used also for
802.1ad bridge.

Move the code to function called mlxsw_sp_bridge_vlan_aware_vxlan_join()
and call it from mlxsw_sp_bridge_8021q_vxlan_join() 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: David S. Miller <davem@davemloft.net>