r8169: enable EEE at 2.5G per default on RTL8125B

Register a6d/12 is shadowing register MDIO_AN_EEE_ADV2. So this line
disables advertisement of EEE at 2.5G. Latest vendor driver r8125
doesn't do this (any longer?), so this mode seems to be safe.
EEE saves quite some energy, therefore enable this mode per default.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Message-ID: <95dd5a0c-09ea-4847-94d9-b7aa3063e8ff@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>