ata: define ATA_PROT_* in terms of ATA_PROT_FLAG_*

This avoid the need to always translate between the two in ata_prot_flags
and generally cleans up the taskfile protocol usage.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tejun Heo <tj@kernel.org>