mlxsw: spectrum_switchdev: Avoid returning errors in commit phase

Drivers are not supposed to return errors in switchdev commit phase if
they returned OK in prepare phase. Otherwise, a WARNING is emitted.
However, when the offloading of a VXLAN tunnel is triggered by the
addition of a VLAN on a local port, it is not possible to guarantee that
the commit phase will succeed without doing a lot of work.

In these cases, the artificial division between prepare and commit phase
does not make sense, so simply do the work in the prepare phase.

Fixes: d70e42b2

 ("mlxsw: spectrum: Enable VxLAN enslavement to VLAN-aware bridges")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>