staging: comedi: comedidev.h: add comments to spin-lock and mutex

Fix the checkpatch.pl issues:

CHECK: spinlock_t definition without comment
CHECK: struct mutes definition withoug comment

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>