net: dsa: make the FDB 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_fdb_prepare and port_fdb_add for simplicity
and convenience. If an hardware error occurs during the commit phase,
there is no need to report it outside the DSA driver itself.

Make the DSA port_fdb_add routine return void for explicitness.

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