link: fix non-vehicle heartbeat filter

[durka]

If there are multiple components sending heartbeats, MAVProxy can get confused, lock onto the wrong one, and fail to receive any messages from the real autopilot.

There is a function in pymavlink, probably_vehicle_heartbeat, which can check for this since this commit ArduPilot/pymavlink@67669c3.

However, MAVProxy does not use probably_vehicle_heartbeat (at least not for this purpose), having its own version called heartbeat_is_from_autopilot in the link modoule.

The corresponding check is present in that function! However (x2), it incorrectly compares an integer against a string, which never matches.

This PR changes the strings to integer constants.

Disclaimer: I am employed by Exyn Technologies, an Ardupilot partner

[peterbarker]

LGTM.

Various places we take strings like this and manipulate them into the relevant integer enumeration value. I guess I might have copied this from one of those...

[peterbarker]

Merged, thanks!