Allow adjustment of IF bandwidth.

[darksidelemm]

After some interesting discussions with the Airspy developer on Twitter, he pointed at some source code from LinRad which provides information on what registers to set (using airspy_r820t_write) to adjust the R820T2's IF bandwidth.

See here for the register values (and corresponding bandwidths): https://sourceforge.net/p/linrad/code/HEAD/tree/trunk/airspy.c#l383
and here for the register addresses: https://sourceforge.net/p/linrad/code/HEAD/tree/trunk/airspy.c#l968

Allowing adjustment of IF bandwidth gives another lever to adjust in optimising receive performance. It would be nice if all the R820T2 registers were documented somewhere, but I suspect the full documentation is only available under NDA, so making use of what information is available from other open source projects will have to do!

I guess the SoapySDR set/listBandwidth functions would be the correct place to expose these settings?

[guruofquality]

Sounds right, can you make a PR?