[PATCH] libata: make ata_set_mode() responsible for failure handling

Make ata_set_mode() responsible for determining whether to take port
or device offline on failure.  ata_dev_set_xfermode() and
ata_dev_set_mode() indicate error to the caller instead of disabling
port directly on failure.  Also, for consistency, ata_dev_present()
check is done in ata_set_mode() instead of ata_dev_set_mode().

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