iio: chemical: atlas-ph-sensor: use regmap_bulk_read

Replaced i2c_smbus_read_i2c_block_data() with regmap_bulk_read()
function call. This is to make the driver code more consistent.

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>