ata: add/use ata_taskfile::{error|status} fields

Add the explicit error and status register fields to 'struct ata_taskfile'
using the anonymous *union*s ('struct ide_taskfile' had that for ages!) and
update the libata taskfile code accordingly. There should be no object code
changes resulting from that...

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>