staging:iio:hwmon bridge: Use iio_read_channel_processed

Use the iio_read_channel_processed function to read the sample value in the
proper unit instead of using iio_read_channel_raw and iio_read_channel_scale and
doing the unit conversion manually.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>