libata: Fix ata_err_string()

Add proper error string output for ATA_ERR_NCQ and ATA_ERR_NODEV_HINT
instead of returning "unknown error".

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>