plugins/fmcomms2_adv: FMCOMMS5 use iio_channel functions with the DDS

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
plugins/fmcomms2_adv: FMCOMMS5 further enhance bisect algo

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>