s390/zcrypt: fix ap_instructions_available() returncodes

During review of KVM patches it was complained that the
ap_instructions_available() function returns 0 if AP
instructions are available and -ENODEV if not. The function
acts like a boolean function to check for AP instructions
available and thus should return 0 on failure and != 0 on
success. Changed to the suggested behaviour and adapted
the one and only caller of this function which is the ap
bus core code.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>