rsxx: Restructured DMA cancel scheme.

Before, DMAs would never be cancelled if there was a data stall
or an EEH Permenant failure which would cause an unrecoverable
I/O hang.

The DMA cancellation mechanism has been modified to fix
these issues and allows DMAs to be cancelled during the
above mentioned events.

Signed-off-by: Philip J Kelleher <pjk1939@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>