fmcomms2_adv.glade: Add RX->TX Loopback

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
fmcomms2_adv.glade: Add Rx->Tx Loopback

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
plugins/fmcomms2: Add RF input/output port select option

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
plugins/fmcomms2: Display RX/TX rates in MHz/MSPS

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