Avoid repeated writes into the shift register

May reduce flickering of LEDs on some boards

src/modules/leds.cpp

@@ -49,8 +49,10 @@ void LEDs::Step() {
         result <<= 1;
         result |= leds[i].Step(oddPeriod);
     }
-
-    hal::shr16::shr16.SetLED(result);
+    if (result != cachedState) {
+        hal::shr16::shr16.SetLED(result);
+        cachedState = result;
+    }
 }
 
 } // namespace leds

src/modules/leds.h

@@ -130,6 +130,9 @@ private:
     /// [8] - green LED slot 4
     /// [9] - red LED slot 4
     LED leds[ledPairs * 2];
+
+    /// Cache for avoiding duplicit writes into the shift registers (may reduce LED flickering on some boards)
+    uint16_t cachedState = 0;
 };
 
 /// The one and only instance of FINDA in the FW