[PATCH] libata: implement ata_dev_disable()

This patch implements ata_dev_disable() which prints a warning message
and takes @dev offline.  Currently, this is done by explicitly
incrementing dev->class with case-by-case warning messages.  Giving
user clear indication when libata gives up will be more important as
libata will be doing more retries.

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