Input: soc_button_array - get rid of MAX_NBUTTONS

Count how much gpio_keys we actually need, this is a preparation patch
for adding support for the new Win10 / ACPI-6.0 "Generic Buttons Device"
support.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>