Input: soc_button_array - Propagate error from gpiod_count()

Since gpiod_count() does not return 0 anymore, we don't need to shadow
its error code and would safely propagate to the user.

While here, replace second parameter by NULL in order to prevent side
effects on _DSD enabled firmware.

Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>