Make setting sync_source to anything other than default trigger a sync to PPS.

[alex-orange]

See title.

[xavierarteaga]

Hi,
Many thanks for opening this pull request.

From the patch, I understand the following condition to reset the time:

1. Any of the following conditions:
   a. Any sync source that is not default; or
   b. More than 1 Rx port; or
   c. More than 1 Tx port. and
2. The sync source is not GPSDO.

If default is set to internal for point a, setting the sync to internal would trigger a time reset. Based on the same configuration, I find this to be a behavior change. What do you think about changing the first point for the synchronization source set to external?

[alex-orange]

Your analysis of what it does is correct. My way of thinking about it is: if the user expressly states a time sync source than something should be done with the PPS (in the case of GPSDO this is done elsewhere). Setting the sync source without setting the time based off of PPS is pointless as it doesn't actually do anything. Therefore I conclude that if the user hasn't specified anything (or has explicitly specified the default) PPS can be ignored, otherwise the time should be set based off of PPS.