mlxsw: spectrum_span: Move SPAN operations out of global file

The per-ASIC SPAN operations are relevant to the SPAN module and
therefore should be implemented there and not in the main driver file.
Move them.

These operations will be extended later on.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>