libata: don't whine if ->prereset() returns -ENOENT

->prereset() returns -ENOENT to tell libata that the port is empty and
reset sequencing should be stopped.  This is not an error condition.
Update ata_eh_reset() such that it sets device classes to ATA_DEV_NONE
and return success in on -ENOENT.  This makes spurious error message
go away.

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