net: dhcpv4: Network interface netmask was set too early

When we receive the subnet mask option from the server, we
cannot yet set the netmask to the network interface as the
mask is tied to the IP address we received from the server.
We need to delay the setting of netmask until we have added
the requested IP address to the interface.

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