Bluetooth: Mesh: Fix rejecting invalid remote public key

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).

This fixes issue that d4fd2670
supposed to fix. The problem is bt_dh_key_gen is async.
Local public key cannot be sent from the same context
the bt_dh_key_gen is called because we don't know yet
if remote key is valid.

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

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>