net: usb: lan78xx: Use ethtool_op_get_link to reflect current link status

Replace the custom lan78xx_get_link implementation with the standard
ethtool_op_get_link helper, which uses netif_carrier_ok to reflect
the current link status accurately.

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/20250618122602.3156678-4-o.rempel@pengutronix.de


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