PlutoSDR incorrectly starts with hardware AGC enabled

[argilo]

If the "Hardware AGC" setting in Gqrx is turned off, PlutoSDR still starts with hardware AGC turned on, because that's the hardware's default behaviour and SoapyPlutoSDR doesn't override it. I've proposed to change that in https://github.com/pothosware/SoapyPlutoSDR/pull/49. If that change is accepted, then this bug will be solved. Otherwise, Gqrx will need to be changed to explicitly set the hardware AGC setting in all cases.