V4L/DVB (9770): em28xx: turn off tuner when not used

em28xx devices generally get hot when xc3028 tuner is powered on. This
patch solves this by turning power off when the device is not used, at the
expense of having a higher load time, when calling a TV application.

Since firmware load happens on 1 or 2 seconds on most devices, this is not a pain.

Also, it helps to save the planet by saving some power :)

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>