Fix reseting CHDK pin to LOW

chdkActive was set to false regardless of (millis() - chdkHigh) being bigger than the CHDK_DELAY or not. So if (millis() - chdkHigh) wasn't bigger than the delay the first time, the CHDK would never be set back to LOW.

Also, don't return from the function, as there might be other stuff to do, after the CHDK check.
Merge pull request #1034 from DerFlob/Marlin_v1

Fix reseting CHDK pin to LOW