[media] af9015: fix and refactor i2c adapter algo logic

* fix write+read when write has more than one byte
* remove lock, not needed on that case
* remove useless i2c msg send loop, as we support only write, read and
write+read as one go and nothing more

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