Bluetooth: Mesh: Fix rejecting invalid remote public key

Mesh Profile Spec v1.0.1 | Section 5.4.2.3:
"The Provisioner and the device shall check whether the public key
provided by the peer device or obtained OOB is valid (see Section
5.4.3.1).

When the Provisioner receives an invalid public key, then provisioning
fails, and the Provisioner shall act as described in Section 5.4.4.
When the device receives an invalid public key, then provisioning
fails, and the device shall act as described in Section 5.4.4."

This is also in Erratum 10395 which is Mandatory for Mesh v1.0.

The code was already rejecting the key, however that rejection
happened only after we had already sent our public key as response,
which got interpreted as acceptance by the tester (PTS).

Fixes MESH/NODE/PROV/BI-13-C.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>