net: dsa: mt7530: get rid of mac_port_validate member of mt753x_info

The mac_port_validate member of the mt753x_info structure is not being
used, remove it. Improve the member description section in the process.

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