IIO Device Source/Sink: Update to latest libiio API

Plus some very minor changes.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Simplify FMComms2 Source block using multiple inheritance

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Increase buffer size of the IIO Device Source/Sink blocks

This reduces the chances of having samples dropped.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Added FMComms2 Sink block

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>