ata: libata: revamp ata_get_cmd_descript()

Rename ata_get_cmd_descrip() to ata_get_cmd_name() and simplify
it to return "unknown" instead of NULL.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>