Make the attribute writeable.

Remove the ramp up to logic as its unnecessary, simply set depth.  Add
debug message if depth changed, possibly reducing limit, yet our
outstanding count has yet to catch up with it.
Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>