staging: comedi: mite: document the mite_struct spinlock_t

Add a comment to fix the checkpatch.pl issue:
CHECK: spinlock_t definition without comment

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