plugins/fmcomms2_adv: Fix segmentation faults

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
plugins/fmcomms2: Update widgets before connecting signals

Otherwise, the signals get triggered while the widgets are updated.
This change makes the FMComms2 plugin load between 2x and 3x faster.

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