net: dsa: bcm_sf2: convert to phylink_generic_validate()

Populate the supported interfaces and MAC capabilities for the bcm_sf2
DSA switch and remove the old validate implementation to allow DSA to
use phylink_generic_validate() for this switch driver.

The exclusion of Gigabit linkmodes for MII and Reverse MII links is
handled within phylink_generic_validate() in phylink, so there is no
need to make them conditional on the interface mode in the driver.

Thanks to Florian Fainelli for suggesting how to populate the supported
interfaces.

Link: https://lore.kernel.org/r/3b3fed98-0c82-99e9-dc72-09fe01c2bcf3@gmail.com
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>