staging: comedi: pcmuio: document the spinlock_t variables

Add some comments about the two spinlock_t variables in the private
data. Also, add come comments for the functions that do not need to
lock/unlock the spinlock.

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>