net: dhcpv4: Fix NAK processing in RENEWING state

NAK message should be processed in the RENEWING state just like in
REBINDING, not ignored.

Additinonally, NAK processing should include the rejected IP address
removal from the nework interface, otherwise the IP address will remain
on the interface indefinitely (i. e. until removed manually), which
might disturb further operation.

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