altosui: Disable 'max flight log' config when there are stored flights

altosui: Disable 'max flight log' config when there are stored flights

When flights are stored in flash, the maximum flight log value cannot
be changed as the flight data might need to be moved around in
memory. Check for this case by looking for stored flights and
disabling the combo box when storage is not empty.

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