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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>