net: phy: lan87xx: fix access to wrong register of LAN87xx

The function lan87xx_config_aneg_ext was introduced to configure
LAN95xxA but as well writes to undocumented register of LAN87xx.
This fix prevents that access.

The function lan87xx_config_aneg_ext gets more suitable for the new
behavior name.

Reported-by: Måns Rullgård <mans@mansr.com>
Fixes: 05b35e7e

 ("smsc95xx: add phylib support")
Signed-off-by: Andre Edich <andre.edich@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>