ar71xx: Use private version of ath79_setup_qca955x_eth_cfg for MR900

ar71xx: Use private version of ath79_setup_qca955x_eth_cfg for MR900

The MR900 must unset some bits in ETH_CFG which were set by u-boot to work
correctly under OpenWrt. But the global function
ath79_setup_qca955x_eth_cfg will not unset all of them to increase the
backward compatiblity with older mach-* files. A private (simplified)
version for MR900 can be used instead.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49069 3c298f89-4303-0410-b956-a3cf2f4a3e73