Device stopped to tuning some channels after regmap conversion.
Reason is that regmap_update_bits() works a bit differently for
partially volatile registers than old homemade routine. Return
back to old routine in order to fix issue.

Fixes: 478932b1

Cc: <stable@kernel.org> # 4.2+
Reported-by: Mark Clarkstone <hello@markclarkstone.co.uk>
Tested-by: Mark Clarkstone <hello@markclarkstone.co.uk>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>