Input: soc_button_array - debounce the buttons

The soc_button_array driver was initializing (kzalloc) the
debounce_interval value to 0, leading to no debouncing at all,
while the buttons are simple mechanical switches.

This commit sets debounce_interval to 50ms to avoid spurious button
press reports both on press and release of the button. Note 50ms may
seem like a lot but soc_button_array is typically used with cheap
tablets, with not so great buttons. I tried 10ms on my tablet and it
is not enough, where as 50ms works well.

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