sys/arduino: Bug and style fixes in SPI

- set is_transaction before releasing the SPI bus (and the mutex) to avoid
  data races
- adapt switch indent to coding convention

sys/arduino/SPI.cpp

@@ -90,8 +90,8 @@ void SPIClass::beginTransaction(SPISettings settings)
 
 void SPIClass::endTransaction()
 {
-    spi_release(spi_dev);
     is_transaction = false;
+    spi_release(spi_dev);
     rmutex_unlock(&mut);
 }