r8169: add support for returning tx_lpi_timer in ethtool get_eee

Add support for returning the tx_lpi_timer value to userspace.
This is supported by few chip versions only: RTL8168h/RTL8125/RTL8126

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/4eee9c34-c5d6-4c96-9b05-455896dea59a@gmail.com


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