libata: -ENODEV during prereset isn't an error

During prereset, -ENODEV return from ata_wait_ready() is not an error.
This causes unnecessary bug message on controllers which uses 0xff to
indicate empty port.  Fix it.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>