Split Common: Serial Backlight LED fix (#3586)

quantum/split_common/matrix.c

@@ -340,6 +340,24 @@ void matrix_slave_scan(void) {
     for (int i = 0; i < ROWS_PER_HAND; ++i) {
         serial_slave_buffer[i] = matrix[offset+i];
     }
+#endif
+#ifdef USE_I2C
+#ifdef BACKLIGHT_ENABLE
+    // Read backlight level sent from master and update level on slave
+    backlight_set(i2c_slave_buffer[0]);
+#endif
+    for (int i = 0; i < ROWS_PER_HAND; ++i) {
+        i2c_slave_buffer[i+1] = matrix[offset+i];
+    }
+#else // USE_SERIAL
+    for (int i = 0; i < ROWS_PER_HAND; ++i) {
+        serial_slave_buffer[i] = matrix[offset+i];
+    }
+
+#ifdef BACKLIGHT_ENABLE
+    // Read backlight level sent from master and update level on slave
+    backlight_set(serial_master_buffer[SERIAL_BACKLIT_START]);
+#endif
 #endif
     matrix_slave_scan_user();
 }