s390/zcrypt: switch return type to bool for ap_instructions_available()

Function ap_instructions_available() had returntype int but
in fact returned 1 for true and 0 for false. Changed returntype
to bool.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>