ata: fixup ATA_PROT_NODATA

The taskfile protocol is a numeric value, and can not be ORed.  Currently
this is harmless as the protocol is always zeroed before, but if it ever
has a non-zero value the ORing would create incorrect results.

Signed-off-by: Hannes Reinecke <hare@suse.de>
[hch: updated patch description]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tejun Heo <tj@kernel.org>