staging: bcm2835-audio: fix mutex definition without comment

Fix issue reported by checkpatch:

CHECK: struct mutex definition without comment
FILE: drivers/staging/vc04_services/bcm2835-audio/bcm2835-vchiq.c:14:
Signed-off-by: Adrien Thierry <athierry@redhat.com>
Link: https://lore.kernel.org/r/20220414181622.102049-5-athierry@redhat.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vc04_services/bcm2835-audio/bcm2835-vchiq.c

@@ -11,7 +11,7 @@ struct bcm2835_audio_instance {
 	struct device *dev;
 	unsigned int service_handle;
 	struct completion msg_avail_comp;
-	struct mutex vchi_mutex;
+	struct mutex vchi_mutex; /* Serialize vchiq access */
 	struct bcm2835_alsa_stream *alsa_stream;
 	int result;
 	unsigned int max_packet;