r8169: add generic rtl_set_eee_txidle_timer function

Add a generic setter for the EEE tx idle timer and use it with all
RTL8125/RTL8126 chip versions, in line with the vendor driver.
This prepares for adding EEE tx idle timer support for additional
chip versions.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/39beed72-0dc4-4c45-8899-b72c43ab62a7@gmail.com


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