[media] af9015: fix i2c failures for dual-tuner devices - part 2

Some changes for previous patch I liked to do.
Just move tuner init and sleep to own functions from the demod
init and sleep functions.  Functionality remains still almost the same.

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