net: dhcpv4: Prevent DHCP client from clearing the gateway

In case both, static IP address configuration and DHCP were used, and no
DHCP server was avaliable in the network, clearing the gateway address
rendered the network interface unusable as it's gateway configuration
was cleared.

Prevent this by removing the gateway clearing during the DHCP
inititalization. If the DHCP server is available in the network, the
gateway address will be overwriten after receiving the DHCP OFFER
message or cleared if there's no Router option is avaiable in the
DHCP OFFER message.

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