This patch adds accounting for active CS. Active means that the CS was
submitted to the H/W queues and was not completed yet.

This is necessary to support suspend operation. Because the device will be
reset upon suspend, we can only suspend after all active CS have been
completed. Hence, we need to perform accounting on their number.
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>