s390x: firq: use CPU indexes instead of addresses

Adapt the test to the new semantics of the smp_* functions, and use CPU
indexes instead of addresses.

replace the checks with asserts, the 3 CPUs are guaranteed to be there.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>