rfkill: fix error return code in rfkill_gpio_probe()

Fix to return a negative error code from the error handling
case instead of 0, as returned elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
[fix some indentation on the way]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>