Revert "ar71xx: Clear bits in ath79_setup_qca955x_eth_cfg"

Not all mach-* files set all boards correctly in ETH_CFG. They depend on
some preset values by u-boot which were not previously modified by
ath79_setup_qca955x_eth_cfg. Avoiding to modify them in this function keeps
it backward compatible for these boards.

This reverts commit 119b8ab2c2eac237ec4e9c4d0ed53df22b5c6978.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49072 3c298f89-4303-0410-b956-a3cf2f4a3e73