gpio-button-hotplug: improve gpio button debouncing, verify state changes over multiple polls. fixes spurious failsafe triggers (#13784)

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

git-svn-id: svn://svn.openwrt.org/openwrt@37090 3c298f89-4303-0410-b956-a3cf2f4a3e73