Bluetooth: controller: llcp: fix issue re. version exchange

Complete the remote initiated version exchange if a LL_VERSION_IND is
received while already having responded in an earlier version exchange
procedure.

Clarify comment regarding how to handle this invalid behaviour.

This has been seen when running the LL/CON/CEN/BI-12-C test on EBQ.

Signed-off-by: Thomas Ebert Hansen <thoh@oticon.com>
(cherry picked from commit d9774bd9)