Fix Xilinx SystemACE driver to handle empty CF slot

The SystemACE driver does not handle an empty CF slot gracefully. An
empty CF slot ends up hanging the system. This patch adds a check for
the CF state and stops trying to process requests if the slot is empty.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>