arm: omap: omap_device notifier should use UNBOUND_DRIVER

Removing any omap device always resulted in a crash; turns out
BUS_NOTIFY_DEL_DEVICE is not the last notifier event sent in the
course of removing the device, the correct event is
BUS_NOTIFY_UNBOUND_DRIVER.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>