net: usb: lan78xx: port link settings to phylink API

net: usb: lan78xx: port link settings to phylink API

Refactor lan78xx_get_link_ksettings and lan78xx_set_link_ksettings to
use the phylink API (phylink_ethtool_ksettings_get and
phylink_ethtool_ksettings_set) instead of directly interfacing with the
PHY. This change simplifies the code and ensures better integration with
the phylink framework for link management.

Additionally, the explicit calls to usb_autopm_get_interface() and
usb_autopm_put_interface() have been removed. These were originally
needed to manage USB power management during register accesses. However,
lan78xx_mdiobus_read() and lan78xx_mdiobus_write() already handle USB
auto power management internally, ensuring that the interface remains
active when necessary. Since there are no other direct register accesses
in these functions that require explicit power management handling, the
extra calls have become redundant and are no longer needed.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20250618122602.3156678-5-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>