net: bt: Fix failing to resolve link-local address

This make sure the link-local address of the destination is added to the
nbr cache as that is accessed when calling net_ipv6_prepare_for_send,
this is needed when following RFC 7668 since link-local addresses are
never registered using nbr discovery.

Change-Id: I2bc578d33d1061726d0cbf46e4464df74d79e992
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>