staging: comedi: das1800: document the spinlock

The comedi_device spinlock is used to protect the indirect addressing
selected by the DAS1800_SELECT register. It also prevents races between
the interrupt handler and the analog input (*poll).

Update the comments to make this clear.

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>