staging: iio: ad2s1200: Improve readability with be16_to_cpup

The manual states that the data is contained in the upper 12 bits
of the 16 bits read by spi. The code that extracts these 12 bits
is correct for both be and le machines, but this is not clear
from a first glance.

To improve readability the relevant expressions are replaced
with equivalent expressions that use be16_to_cpup.

Signed-off-by: David Veenstra <davidjulianveenstra@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>