mlxsw: spectrum_switchdev: Move mlxsw_sp_bridge_vxlan_join()

mlxsw: spectrum_switchdev: Move mlxsw_sp_bridge_vxlan_join()

Next patch will call __mlxsw_sp_bridge_vxlan_leave() from
mlxsw_sp_bridge_vxlan_join() as part of error flow, move the function to
be able to call the second one.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/64750a0965536530482318578bada30fac372b8a.1742224300.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>