Bugfix in position decoding

If the aircraft lands or takes off, the Lat/Lon valid flags are cleared.
In the original code, this also resulted in any even/odd position
reports in the mm record being discarded  This meant that the code would
require an even and odd position after the change of flight status.

The code had been modified so that any position report in the mm record
is used even if there is a change of flight status. This means there
only needs to be an even or odd after a change of FS, rather than
requiring both even and odd. The result should be earlier decoding of
position.