net: dsa: mt7530: define MAC speed capabilities per switch model

With the support of the MT7988 SoC switch, the MAC speed capabilities
defined on mt753x_phylink_get_caps() won't apply to all switch models
anymore. Move them to more appropriate locations instead of overwriting
config->mac_capabilities.

Remove the comment on mt753x_phylink_get_caps() as it's become invalid with
the support of MT7531 and MT7988 SoC switch.

Add break to case 6 of mt7988_mac_port_get_caps() to be explicit.

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