[media] af9013: refactor power control

Move power-up and power-down functionality to init/sleep ops and
get rid of old function.

Fixes and simplifies power-up functionality slightly.

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