net: sockets: Fill the address structure provided in recvfrom()

The packet socket implementation did not fill the address structure
provided by the application. This commit fixes this.

Note, that the implementation needs to cover two cases: SOCK_RAW and
SOCK_DGRAM. In the first case, the information is extracted directly
from the L2 header (curently only Ethernet supported). In latter case,
the header is already removed from the packet as the L2 has already
processed the packet, so the information is obtained from the net_pkt
structure.

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