gpio-button-hotplug: fix wrong initial seen value

gpio-button-hotplug: fix wrong initial seen value

Currently the generated event contains wrong seen value, when the button
is pressed for the first time:

 rmmod gpio_button_hotplug; modprobe gpio_button_hotplug
 [ pressing the wps key immediately after modprobe ]
 gpio-keys: create event, name=wps, seen=1088, pressed=1

So this patch adds a check for this corner case and makes seen=0 if the
button is pressed for the first time.

Tested-by: Kuan-Yi Li <kyli.tw@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>