Device sink block: Fix block leaking samples

It now streams waveforms continuously.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Added a 'fm-transmitter' example using the FMCOMMS2 block

It is advised to run the generated Python script with a negative
niceness and/or with a realtime kernel, to ensure that no
samples are lost.

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