[PATCH] libata: fix ->phy_reset class code handling in ata_bus_probe()

ata_bus_probe() doesn't clear dev->class after ->phy_reset().  This
can result in falsely enabled devices if probing fails.  Clear
dev->class to ATA_DEV_UNKNOWN after fetching it.

Signed-off-by: Tejun Heo <htejun@gmail.com>