ar71xx: fix ethernet initialization on QCA953x-based boards (TP-Link...

ar71xx: fix ethernet initialization on QCA953x-based boards (TP-Link TL-WR841N/ND v9, Compex WPJ531)

The initialization routines for these boards were relying on some (wrong)
defaults for the QCA953x ethernet. Make these defaults explicit to prevent
breaking them when the QCA953x defaults are fixed.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46206 3c298f89-4303-0410-b956-a3cf2f4a3e73