[PATCH] libata: implement ata_dev_absent()

For the time being we cannot use ata_dev_present() as it was renamed
to ata_dev_enabled() but we still need presence test.  Implement
negation of the test.  Conveniently, the negated result is needed in
more places.  This is suggested by Jeff Garzik.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>