mlxsw: Add extack messages for port_{un, }split failures

Return messages in extack for port split/unsplit errors. e.g.,
    $ devlink port split swp1s1 count 4
    Error: mlxsw_spectrum: Port cannot be split further.
    devlink answers: Invalid argument

    $ devlink port unsplit swp4
    Error: mlxsw_spectrum: Port was not split.
    devlink answers: Invalid argument

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>