[IPV4]: ipconfig.c: fix dhcp timeout behaviour

I think there is a small bug in ipconfig.c in case IPCONFIG_DHCP is set
and dhcp is used.

When a DHCPOFFER is received, ip address is kept until we get DHCPACK.
If no ack is received, ic_dynamic() returns negatively, but leaves the
offered ip address in ic_myaddr.

This makes the main loop in ip_auto_config() break and uses the maybe
incomplete configuration.

Not sure if it's the best way to do, but the following trivial patch
correct this. 

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>