SSB modulator fix
Set samples to zero when no modulation is active. This fixes remaining DC leaving a carrier with no modulation. Fixes issue #204
Web API fixes
- fixed bug in PUT/PATCH of modulators not setting differentially
- fixed TestSource device setting
- AM demod: added pll and syncAMOperation settings processing
- implemented instanceDVSerialGet
- RTL-SDR: fixed RF bandwidth setting
Other changes
- WFM demod: corrected audio mute display
- Fixed power display going to floor value in some demods
- RTL-SDR: fixed low sample rate setting