Bluetooth: Host: Fix SMP Pairing failed code on invalid Public Key

Core Specification 6.1 clarified exptected erro code in case peer
sending invalid Public Key. In case pairing is aborted during or
immediately after Public Key Exchange phase has completed (which is
the case here) expected reason code is set to "DHKey Check Failed".

This was affecting SM/CEN/KDU/BI-04-C and SM/PER/KDU/BI-04-C
qualification test cases.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>