net: phylink: Use phy_caps to get an interface's capabilities and modes

Phylink has internal code to get the MAC capabilities of a given PHY
interface (what are the supported speed and duplex).

Extract that into phy_caps, but use the link_capa for conversion. Add an
internal phylink helper for the link caps -> mac caps conversion, and
use this in phylink_caps_to_linkmodes().

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250307173611.129125-14-maxime.chevallier@bootlin.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>