net: phylink: provide mac_get_caps() method

Provide a new method, mac_get_caps() to get the MAC capabilities for
the specified interface mode. This is for MACs which have special
requirements, such as not supporting half-duplex in certain interface
modes, and will replace the validate() method.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qsPk5-009wiX-G5@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>