ralink: add port_init for rt2880

Forgotten patch to fix ethernet for rt2880.
Based on previously submitted patch from Claudio.

Signed-off-by: Roman Yeryomin <roman@advem.lv>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40504 3c298f89-4303-0410-b956-a3cf2f4a3e73