s390/zcrypt: enable AP bus scan without a valid default domain

The AP bus scan is aborted before doing anything worth mentioning if
ap_select_domain() fails, e.g. if the ap_rights.aqm mask is all zeros.
As the result of this the ap bus fails to manage (e.g. create and
register) devices like it is supposed to.

Let us make ap_scan_bus() work even if ap_select_domain() can't select a
default domain. Let's also make ap_select_domain() return void, as there
are no more callers interested in its return value.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reported-by: Michael Mueller <mimu@linux.ibm.com>
Fixes: 7e0bdbe5 "s390/zcrypt: AP bus support for alternate driver(s)"
[freude@linux.ibm.com: title and patch header slightly modified]
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>