libata: Fix sysfs documentation bug

forced_unqueued indicates that a drive's queued DSM support is
broken. Update sysfs ABI documentation accordingly.

Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>