kernel: fix config breakage from removing the de4x5 tulip driver

Properly define the config symbol to n so it doesn't break build for
targets which do have it available (e.g. mostly everything except arm).

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37489 3c298f89-4303-0410-b956-a3cf2f4a3e73