kernel: move a symbol from the ipq806x config to generic to prevent it from being nuked by make kernel_oldconfig

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45920 3c298f89-4303-0410-b956-a3cf2f4a3e73