net: if: Fix typo in IPv6 hop limit API name

The net_if_ipv6_set_hop_limit() API was missing the "_if_"
part in it. Fix this so that the network interface API is
consistent. The old function is deprecated and should not
be used. The old function is left to the code and it calls
the new properly named function.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>