icE1usb fw/gpsdo: Use PID loop instead of ad-hoc algo

The original algo is just somehting I originally came up with
on the spot and worked and I never got the time to revisit.

Now after some testing, I implemented a PID loop that seems to
present faster lock time and at least as good stability.

Parameters were not thorougly optimized just some 'hand' tuning
trying to simulate the behavior.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Change-Id: Ie3ea7243aa4234585f1ace222632bb5580caca75
1 file changed