net: usb: lan78xx: remove unused struct members

Remove unused members from struct lan78xx_net, including:

    driver_priv
    suspend_count
    mdix_ctrl

These fields are no longer used in the driver and can be safely removed
as part of a cleanup.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20250618122602.3156678-7-o.rempel@pengutronix.de


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