Fix for Airspy MIXER gain

So I was having a problem with my GOES LRIT receiver in GNU Radio. In GQRX the SNR was 7dB and in GNU Radio was 4dB regardless of the setting. Then I noticed that the Mixer (BB Gain) wasn't working at all. Checked over the internet and there was this issue: So I made some changes and here is the working version: https://github.com/osmocom/gr-osmosdr/pull/8 (I know its a mirror, but the patch is also attached) Lucas Teske