libata: return boolean values from ata_is_*

This way we don't have to worry about the exact bit postition of the
test to leak out and any crazy propagation effects in the callers.

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