[media] em28xx: add debouncing mechanism for GPI-connected buttons

So far, the driver only supports a snapshot button which is assigned to
register 0x0c bit 5. This special port has a built-in debouncing mechanism.
For buttons connected to ordinary GPI ports, this patch implements a software
debouncing mechanism.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>