Several fixes and enhancements for ldc1612

An assortment of fixes and enhancements:

Fix timing during probe result analysis which could lead to "Unable to obtain probe_eddy_current sensor readings" errors (as reported by @Arksine).
Fix _start_from_home typo (fix from @Arksine).
Improve calibration on typical printers by probing every 40um (instead of 50um).
Improve support for chips with INTB line routed to mcu.
Propagate an error if the chip reports a sensing error (previously a warning would be incorrectly propagated as a trigger).

-Kevin

Klipper3d / klipper