Add symbolic constants SETTLE_DEVICE_*_MAX_WAIT_SECONDS

Make the code a little more self documenting by adding the symbolic
constants:
    SETTLE_DEVICE_APPLY_MAX_WAIT_SECONDS
    SETTLE_DEVICE_PROBE_MAX_WAIT_SECONDS
which highlight that settle_device() is called in two different
contexts, device probe and apply operations, with two different timeout
values.