[PATCH] libata: make ata_bus_probe() return negative errno on failure

ata_bus_probe() uses unsigned int rc to receive negative errno and
returns the converted unsigned int value.  Convert temporary variables
to int and make ata_bus_probe() return negative errno on failure.

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