Reduce cal_max 1220->1180 to absorb ADC jitter at toe

Raw ADC jitter of ~40 counts at toe caused MIDI flickering
between 127 and ~123. Lowering cal_max so the >= cal_max
clamp at 127 absorbs the noise.

src/expression_pedal.cpp

@@ -11,7 +11,7 @@ ExpressionPedal::ExpressionPedal(uint8_t adc_pin)
     , last_read_time(0)
     , current_raw(0)
     , cal_min(36)
-    , cal_max(1220)
+    , cal_max(1180)
 {
 }