include: net: net_pkt: Use IEEE802154 instead of NET_L2_IEEE802154

Use CONFIG_IEEE802154 instead of CONFIG_NET_L2_IEEE802154
and CONFIG_IEEE802154_RAW_MODE when adding RSSI and LQI
members to net_pkt. CONFIG_IEEE802154 is selected when
at least one 15.4 device is selected (L2 or RAW).

Signed-off-by: Kamil Sroka <kamil.sroka@nordicsemi.no>