r8169: factor out rtl8168h_2_get_adc_bias_ioffset

Simplify and factor out this magic from rtl8168h_2_hw_phy_config()
and name it based on the vendor driver.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>