[PATCH] libata: fix missing classes[] initialization in ata_bus_probe()

ata_bus_probe() didn't initialize classes[] properly with
ATA_DEV_UNKNOWN.  As ->probe_reset() is allowed to leave @classes
alone when no device is present, this results in garbage class values.
ATM, the only affected driver is ata_piix.

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