use atomic ops to increment and decrement the device ref count in device_ref and device_unref.
use atomic ops to increment and decrement the device ref count in
device_ref and device_unref.

ok guenther@ deraadt@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars