Input: gpio-keys - fix possible NULL pointer dereference

bdata->button is used in gpio_check_button but never initialized.  Having a
device with debounce_interval != 0 without this patch resulted on an oops on
my machine.

Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>