ata: libata-eh: Use ata_ncq_enabled() in ata_eh_speed_down()

In ata_eh_speed_down(), instead of hard-coding the test on the device
flags to detect if NCQ is supported and enabled, use ata_ncq_enabled().

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>