kernel: disable kmod-iio for linux 3.6 as well (to avoid build issues due to missing config symbol changes)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38247 3c298f89-4303-0410-b956-a3cf2f4a3e73