spi mutex only blocked when gpio != NO_GPIO

linux/spi/SpiComIF.h

@@ -39,6 +39,11 @@ public:
             size_t requestLen) override;
     ReturnValue_t readReceivedMessage(CookieIF *cookie, uint8_t **buffer,
             size_t *size) override;
+    /**
+     * @brief   This function returns the mutex which can be used to protect the spi bus when
+     *          the chip select must be driven from outside of the com if.
+     */
+    MutexIF* getMutex();
 private:
 
     struct SpiInstance {