gpio-button-hotplug: debounce the initial button state, the first reads at boot time might be wrong

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

git-svn-id: svn://svn.openwrt.org/openwrt@37702 3c298f89-4303-0410-b956-a3cf2f4a3e73