Add support for 16-bit complex samples

[guruofquality]

16-bit samples are used by a lot of applications, but currently only floating point is supported (widely used by osmosdr blocks, gqrx, etc). Looking for someone with AirspyHF to test and develop this. SoapyAirspy driver largely has a good example of this to pull from.

[ast]

I plan to start improving this driver. I will address this issue then.