staging: comedi: pcmuio: spinlock protect pcmuio_{write, read}()

Currently only the pcmuio_handle_asic_interrupt() function uses the
spinlock in the private data to protect the read of the paged interrupt
id registers. All accesses to the paged registers should be protected
to ensure that the page is not changed until the access is complete.
Move the lock/unlock into the pcmuio_{write,read}() functions to make
sure the access completes correctly. Rename the spinlock to variable
to clarify its use.

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