staging: comedi: pcmmio: rename 'spinlock' in the private data

This spinlock_t is meant to protect the page registers in the asic.
Rename it 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>