[PATCH] libata: allow ->probe_reset to return ATA_DEV_UNKNOWN

This patch makes ata_bus_probe() normalize classes[] returned by
->probe_reset such that ->probe_reset can return ATA_DEV_UNKNOWN.
This eases implementation of ->probe_reset's which don't directly use
ata_drive_probe_reset().

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>