net: mscc: ocelot: change prototypes of switchdev port attribute handlers

This is needed so that the Felix DSA front-end can call the Ocelot
implementations.

The implementation of the "mc_disabled" switchdev attribute has also
been simplified by using the read-modify-write macro instead of
open-coding that operation.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>