net/ieee802154: Use pre-filled link address as ll target

IPv6 fills in lladdr_dst properly according to nexthop. Thus it's
useless to lookup again for nbr when creating data frame etc... Let's
use this directly. It both optimizes the code path as well as implements
proper nexthop frame sending.

Jira: ZEP-1863

Change-Id: Ic17a1b14a0db0692d31419ea2f45a5f288a09fb5
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>