V4L/DVB: gspca - sn9c20x: use gspca's input device handling

Drop custom code for handling the input button in
favor of using gspca's input hanlding mechinism.

Signed-off-by: Brian Johnson <brijohn@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>