[PATCH] libata: make ata_set_mode() handle no-device case properly

Make ata_set_mode() return without doing anything if there is no
device on the port.  This is in preparation for ata_bus_probe()
changes.

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