Input: soc_button_array - bail out earlier if gpiod_count is zero

The PNP0C40 device of the Surface 3 doesn't have any GPIO attached to it.
Instead of trying to access the GPIO, request the count beforehand and
bail out if it is null or if an error is returned.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>