Don't Switch to DCM to get Better GPS if DCM Isn't Using GPS

[njwhite]

Look at the parameter to see if DCM uses GPS; if not don't prefer DCM for using GPS.

[tridge]

@njwhite I've rebased this, and fixed the commit comment for our style guidelines. Good change, thanks!
please also see the recent addition of AHRS_OPTIONS that gives you control over the DCM fallback
#25676

[njwhite]

@tridge CI has passed - could you merge please? Thanks -

[IamPete1]

Need to drop the merge commit

Commit msg fixedup and merge commit removed.

[njwhite]

@tridge @IamPete1 thanks for you help getting this merged!