cleanup: use an enum for the error states of nr_cpus_mask

cleanup: use an enum for the error states of nr_cpus_mask

Using an enum with labels for error states instead of literal values
will make the code easier to read and understand.

Change-Id: I4558e17ccb45ab40515bb516af840b2852ee8fc3
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>