plugins/dac_data_manager.c: set/get only existing "dds mode" comboboxes
Some devices such as ad9364 have only one "dds mode" combobox and the dac_data_manager should be aware of this.

Signed-off-by: Dan Nechita <dan.nechita@analog.com>
plugins/fmcomms2.c: Fix upper limit of sampling frequency spinbuttons
Set maximum sampling frequency to 61.44 MHz for FMComms2/3 and 122.88 for FMComms4
fmcomms2.glade: Put back the action widget of the calibration dialog
Glade manages to delete this widget every time I modify and save this file. I'm using 3.12.1 version which is designed for GTK+3. I should consider using Glade for GTK+2 (version 3.8).

Signed-off-by: Dan Nechita <dan.nechita@analog.com>
plugins/fmcomms1.c: Run a FFT plot in the calibration dialog background
The fmcomms1 calibration requires the iio-oscilloscope to have a complex FFT plot running with image markers set.

Signed-off-by: Dan Nechita <dan.nechita@analog.com>