net: dsa: mt7530: explain exposing MDIO bus of MT7531AE better

Unlike MT7531BE, the GPIO 6-12 pins are not used for RGMII on MT7531AE.
Therefore, the GPIO 11-12 pins are set to function as MDC and MDIO to
expose the MDIO bus of the switch. Replace the comment with a better
explanation.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: David S. Miller <davem@davemloft.net>