ar71xx: Use private version of ath79_setup_qca955x_eth_cfg for MR1750

The MR1750 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 MR1750 can be used instead.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>

SVN-Revision: 49070