ar71xx: Set eth1 MAC address from EEPROM for WNDR3700

ar71xx: Set eth1 MAC address from EEPROM for WNDR3700

The WNDR3700's art/caldata partition contains three MAC addresses at
offsets 0, 6, and 12, used for eth0/wlan0 (shared), eth1, and wlan1,
respectively. The existing code configures eth1's MAC address by adding
1 to eth0's instead of using the value stored in the EEPROM. This patch
causes the stored value to be used directly. (On my WNDR3700s, the three
stored addresses are all sequential anyway.)

Signed-off-by: Mark Mentovai <mark@moxienet.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@23876 3c298f89-4303-0410-b956-a3cf2f4a3e73