iio: Fix sink scalar value to match format le:S16/16.

[AlexandraTrifan]

Pull Request Details

Fix scalar value in fmcomms2 sink to match the IIO format le:S16/16>>0

Description

TX output power is aprox 25 dB lower (for the given example) when using ADALM Pluto with gnuradio 3.10 (compared to 3.8 results).
There is an error when converting data from float to int.
The data is scaled to 12 bits instead of 12 bits left shifted by 4 (16 bits where the 4 LSBs don't matter).
This caused the shifting of 4 most significant bits to the right, essentially losing 4 MSBs where most of the power is (about 90% power).

Related Issue

Reported and tested here: https://ez.analog.com/adieducation/university-program/f/q-a/560641/pluto-grc-3-10-sink-block-has-low-output-power .

Which blocks/areas does this affect?

The fmcomms2 sink, part of the gr-iio module.

Testing Done

Testing reported in the issue mentioned above.

Checklist

• I have read the CONTRIBUTING document.
• I have squashed my commits to have one significant change per commit.
• I have signed my commits before making this PR
• My code follows the code style of this project. See GREP1.md.
• I have updated the documentation where necessary.
• I have added tests to cover my changes, and all previous tests pass.