iio: chemical: atlas-ph-sensor: switch regmap cache

switch from using REGCACHE_FLAT to REGCACHE_RBTREE so initial hw values
are read from device. This also allows some volatile ranges to be
dropped.

Note that REGCACHE_FLAT is intended only for very low lag cases so doesn't
do nice things like read initial values from the device.  Hence this change.
Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/chemical/atlas-ph-sensor.c

@@ -65,8 +65,6 @@ struct atlas_data {
 
 static const struct regmap_range atlas_volatile_ranges[] = {
 	regmap_reg_range(ATLAS_REG_INT_CONTROL, ATLAS_REG_INT_CONTROL),
-	regmap_reg_range(ATLAS_REG_CALIB_STATUS, ATLAS_REG_CALIB_STATUS),
-	regmap_reg_range(ATLAS_REG_TEMP_DATA, ATLAS_REG_TEMP_DATA + 4),
 	regmap_reg_range(ATLAS_REG_PH_DATA, ATLAS_REG_PH_DATA + 4),
 };
 
@@ -83,7 +81,7 @@ static const struct regmap_config atlas_regmap_config = {
 
 	.volatile_table = &atlas_volatile_table,
 	.max_register = ATLAS_REG_PH_DATA + 4,
-	.cache_type = REGCACHE_FLAT,
+	.cache_type = REGCACHE_RBTREE,
 };
 
 static const struct iio_chan_spec atlas_channels[] = {