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.h:64:
Signed-off-by: Adrien Thierry <athierry@redhat.com>
Link: https://lore.kernel.org/r/20220414181622.102049-6-athierry@redhat.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/vc04_services/bcm2835-audio/bcm2835.h

@@ -61,7 +61,7 @@ struct bcm2835_chip {
 
 	unsigned int opened;
 	unsigned int spdif_status;
-	struct mutex audio_mutex;
+	struct mutex audio_mutex; /* Serialize chip data access */
 
 	struct bcm2835_vchi_ctx *vchi_ctx;
 };