gpio-button-hotplug: fix crash on remove

Don't call gpio_keys_remove recursively. Setting the platform
data to NULL triggered an oops on the second iteration, so there was
no infinate loop.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39124 3c298f89-4303-0410-b956-a3cf2f4a3e73