ata: Documentation, fix function names

ata_qc_prep no longer exists, there are ata_bmdma_qc_prep and
ata_bmdma_dumb_qc_prep instead. And most drivers do not use them, so
reword the paragraph.

ata_qc_issue_prot was renamed to ata_sff_qc_issue. ->tf_load is now
->sff_tf_load. Fix them.

And fix spelling supercede -> supersede.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-ide@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>