gpio-button-hotplug: handle EPROBE_DEFER and other errors

of_get_gpio_flags() could return an error like EPROBE_DEFER which was
not handled before. This patch takes the code from gpio_keys_polled.c
for error handling and also improves some other unrelated small parts.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46502 3c298f89-4303-0410-b956-a3cf2f4a3e73