s390/vfio-ap: Fix memory allocation for mdev_types array

s390/vfio-ap: Fix memory allocation for mdev_types array

The vfio-ap crypto driver fails to allocate memory for an array of
pointers used to pass supported mdev types to mdev_register_parent().

Since we only support a single mdev type, the fix is to allocate a
single entry in the ap_matrix_dev->mdev_types array.

Link: https://lore.kernel.org/r/20221021145905.15100-1-jjherne@linux.ibm.com
Fixes: da44c340c4fe ("vfio/mdev: simplify mdev_type handling")
Cc: stable@vger.kernel.org
Cc: Tony Krowiak <akrowiak@linux.ibm.com>
Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Jason J. Herne <jjherne@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>