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

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 <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Acked-by: Jiri Pirko <[email protected]>
Signed-off-by: David S. Miller <[email protected]>