mlxsw: core: Extend driver ops by remove selected ports op

In case of line card implementation, the core has to have a way to
remove relevant ports manually. Extend the Spectrum driver ops by an op
that implements port removal of selected ports upon request.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>