Though variable net_dhcp_server_ip was declared and used as a
function parameter it was never assigned. That's why a DHCP
request was sent without option 54 set. Some DHCP server for
example dnsmasq didn't accept such a packet.

As both offered IP address and server IP are stored in the global
dhcp_result structure we don't need either net_dhcp_server_ip or
OfferedIP variables and can use the addresses from dhcp_result
directly.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>