ramips: fix not set lan/wan mac address

The first switch case is default case. Then it will only
match default case. cause not update the lan/wan mac address.
And in the default switch case the param lan_mac is empty.
it will case error message 'arithmetic syntax error' when
call macaddr_add function.

Signed-off-by: michael lee <igvtee@gmail.com>

SVN-Revision: 40910