Just continue without constraints checking in this case.

To this end, remove the 'constraints_populated' field from 'struct
acpi_spmc_softc' and any reference to it.

However, we introduce another boolean, 'sc->get_constraints_succeeded',
in order to check (under INVARIANTS) that acpi_spmc_get_constraints() is
called only once on success. Calling that function another time after
a success would leak memory. It would be easy to change that function
to support multiple calls (e.g., by adding a call to
acpi_spmc_free_constraints() near its start), however trying to retrieve
the constraints again simply looks like wasted time as the same results
are expected to be returned on each call.