net: dsa: make the VLAN add function return void

The switchdev design implies that a software error should not happen in
the commit phase since it must have been previously reported in the
prepare phase. If an hardware error occurs during the commit phase,
there is nothing switchdev can do about it.

The DSA layer separates port_vlan_prepare and port_vlan_add for
simplicity and convenience. If an hardware error occurs during the
commit phase, there is no need to report it outside the driver itself.

Make the DSA port_vlan_add routine return void for explicitness.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>