[media] r820t: Set gain mode to auto

This tuner works with 2 modes: automatic gain mode and manual
gain mode. Put it into automatic mode, as we currently don't
have any API for manual gain adjustment.
The logic to allow setting the manual mode is there, as it is
just a few extra code. This way, if/when we latter add support
for setting the gain mode, the code is already there.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Tested-by: Antti Palosaari <crope@iki.fi>