Fix a memset pointer cast (#14823)

Marlin/src/HAL/HAL_SAMD51/Servo_SAMD51.cpp

     SYNC(tc->COUNT16.SYNCBUSY.bit.CTRLB);
 
     // Reset all servo indexes
-    memset(currentServoIndex, 0xFF, sizeof(currentServoIndex));
+    memset((void *)currentServoIndex, 0xFF, sizeof(currentServoIndex));
 
     // Configure interrupt request
     NVIC_ClearPendingIRQ(SERVO_IRQn);