update xsns_60_GPS.ino

[Staars]

Description:

Computation of the fractions of the second for the NTP-server. Can give accuracy of ~100 milliseconds with (very) good WiFi.
In my testing most of the glitches were related to poor WiFi performance and it got a lot better, after upgrading my AP.

Still use sensor60 9 to turn on NTP server. New: This will stop location reading after the first sat fix and only read time packets after that.

New: Set refID to "GPS" and "rootDelay" & "rootDispersion" to some realistic values (I hope).

This is still no professional timeserver ;)

Checklist:

• [x ] The pull request is done against the latest dev branch
• [x ] Only relevant files were touched
• [x ] Only one feature/fix was added per PR.
• [x ] The code change is tested and works on core Tasmota_core_stage
• [x ] The code change pass travis tests. Your PR cannot be merged unless tests pass
• [x ] I accept the CLA.