[ar71xx] fixes the ethernet port mask of RB433 in the platform data

 * Signed-off-by: Joerg Albert <jal2 at gmx.de> 
 * closes #4185

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@13199 3c298f89-4303-0410-b956-a3cf2f4a3e73