staging: comedi: addi_apci_2032: document the spinlock_t definition

Add some comments to the private data tp quiet the checkpatch.pl
issue about:

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>