ads1x1x: Revert incorrect removal of assignment to self.config

Commit d120a313b incorrectly removed an assignment to self.config - in
this instance the reference was to a local variable not related to the
global configfile storage.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/ads1x1x.py

@@ -321,6 +321,7 @@ class ADS1X1X_pin:
     def __init__(self, chip, config):
         self.mcu = chip.mcu
         self.chip = chip
+        self.config = config
 
         self.invalid_count = 0