ide: add ide_pktcmd_tf_load() helper

Add ide_pktcmd_tf_load() helper and convert ATAPI device drivers to use it.

There should be no functionality changes caused by this patch.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>