net: ipv6: Fix Neighbor Advertisement processing w/o TLLA option

According to RFC 4861, ch. 7.2.5:

 "If the Override flag is set, or the supplied link-layer address
  is the same as that in the cache, or no Target Link-Layer Address
  option was supplied, the received advertisement MUST update the
  Neighbor Cache entry as follows

  ...

  If the Solicited flag is set, the state of the entry MUST be
  set to REACHABLE"

This indicates that Target Link-Layer Address option does not need to be
present in the received solicited Neighbor Advertisement to confirm
reachability. Therefore remove `tllao_offset` variable check from the
if condition responsible for updating cache entry. No further changes in
the logic are required because if TLLA option is missing,
`lladdr_changed` will be set to false, so no LL address will be updated.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>