staging:iio:cdc:ad7150: Shift the _raw readings by 4 bits.

Every other register related to raw value on the datasheet is
described as correpsonding to the 12MSB of the actual
data registers + the bottom 4 bits are 0.  So lets treat this
as what it actually is, which is a 12 bit value.
Note that we will have to be a little careful to compensate for
the offset and scale values.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-18-jic23@kernel.org