net: dsa: support use of phylink_generic_validate()

Support the use of phylink_generic_validate() when there is no
phylink_validate method given in the DSA switch operations and
mac_capabilities have been set in the phylink_config structure by the
DSA switch driver.

This gives DSA switch drivers the option to use this if they provide
the supported_interfaces and mac_capabilities, while still giving them
an option to override the default implementation if necessary.

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>