Input: soc_button_array - fix a crash during rmmod

When the system has zero or one button available, trying to rmmod
soc_button_array will cause crash. Fix this by properly handling -ENODEV
in probe().

Signed-off-by: Lejun Zhu <lejun.zhu@linux.intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>