net: dhcpv4: Do not proceed with DHCPv4 when interface is down

It should be possible to configure/initialize the DHCPv4 for an
interface when the interface is down, the DHCPv4 however should not
proceed with the procedure in such case.

Add a simple check to prevent DHCPv4 from sending packets when interface
is down. As the module registers to the NET_IF_UP event, the procedure
will be restarted when the interface goes up.

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