Gain limit in oversampling mode could be potentially be extended

[Aang23]

Hello!

When I implemented support for this new feature in SatDump, I noticed the gain limit was 11. Out of curiosity and to get an idea of the reasoning behind it, I played around with the full gain range unlocked.

It is pretty clear anything above 11 is unusable, but starting from 39db, the spectrum is usable again. It's not perfect, but it's for some usecases better than the sharper cutoff there seems to be under 11db. This provides a similar result to adding an extra LNA (such as a BT200), and in my testing can help a bit with wideband demodulation.

Spectrum with a 50 ohms reference :

Spectrum hooked up onto L-Band 1.5Ghz Inmarsat signals :

My current patch is the following :

    if (dev->feature == BLADERF_FEATURE_OVERSAMPLE && (gain > 11 && gain < 39)) {
        log_warning("Out of oversample feature gain range. RF Gain clamped to 11.\n");
        gain = 11;
    }