telegps: Don't re-add frequency menu when already present.

If the receiver disappears, we'll stop tracking, but won't pull the
frequency menu down. Doing that would take a bit of work, and it
doesn't seem worth the effort. As a kludge-around, avoid re-creating
the frequency menu if it's already displayed when we connect to
another device.

Signed-off-by: Keith Packard <keithp@keithp.com>