AC_WPNav: wpnav speed param check fixed

[rmackay9]

This is an alternative to #23220

This PR fixes an issue where RTL_SPEED = 400 but if users changed the WPNAV_SPEED parameter the vehicle would immediately change to this new speed.

This has been tested in SITL to confirm that user changes to WPNAV_SPEED will only take effect if the WPNAV_SPEED parameter value was used to initialise the vehicle's speed in the current mode. The results are slightly unintuitive in some situations though so I'm happy to discuss if people think the behaviour is odd.

1. Fly in Auto mode, waypoint command, changes to WPNAV_SPEED take effect immediately
2. Fly in Auto mode, RTL command, if RTL_SPEED was zero when command started meaning WPNAV_SPEED was used, changes to WPNAV_SPEED will take effect immediately. If RTL_SPEED was non-zero when command started, WPNAV_SPEED will not take effect. Changes to RTL_SPEED while vehicle is executing RTL never take effect immediately
3. Fly in RTL mode acts just like 2 above

Below are some screenshots of SITL testing because PRs with pictures are nicer to look at.

[lthall]

It isn't something we should think of as anything more than a very temporary fix but it works.

[rmackay9]

I'm merging to save us some time in tomorrow's dev call because I think we've already discussed this issue enough as a team to know the direction forward.