s390/zcrypt: Fix ap_max_domain_id for older machine types

According to the system architecture the current implementation
requires the presence of the N bit in GR2 in the TAPQ response
field to validate the max. number of domains (Nd).
Older machine types don't have this N bit, hence the max. domain
field was ignored.

Before the N bit was introduced the maximum number of domain was
a constant value of 15. So set this value in case of N bit absence.

Signed-off-by: Ingo Tuchscherer <ingo.tuchscherer@linux.vnet.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>