brcm63xx: reduce the number of Ethernet DMA channels from 16 to 8

We currently used 16 DMA Ethernet channels, but the hardware only
supports up to 8 (with 2 per enet and the remaining for USB slave).

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36979 3c298f89-4303-0410-b956-a3cf2f4a3e73