qml/SignalRow: Hide frequency control entirely when not needed
qml/SignalRow: Hide frequency control entirely when not needed

When hidding the frequency input widget it is not enough to set the text
to "" because it is still possible to click on the widget making the edit
cursor to be displayed.

Signed-off-by: Dan <2f664a27dbc3ee2034ed9e950b3808890cb5416a@analog.com>

analogdevicesinc/Pixelpulse2,analogdevicesinc/Pixelpulse2,analogdevicesinc/Pixelpulse2