Input: gpio-keys - report error when disabling unsupported key

When trying to disable a key that is not supported by the device we should
report error, not do nothing and report success:

root@yocto:/sys/devices/soc0/gpio-keys# cat keys
114-116
root@yocto:/sys/devices/soc0/gpio-keys# echo 77 > keys
root@yocto:/sys/devices/soc0/gpio-keys#

We want 'echo 77 > keys' to report an error, but silence to give us an
illusion that all is 'ok'.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>