net: dhcpv4: client: Do not generate new xid for Request message

According to RFC 2131, DHCP clients should use the same xid as
received in the Offer message when sending DHCP Requests. Therefore,
when generating DHCP Request message, the xid value should not be
incremented.

One vague topic is whether the xid value should be updated when
sending Requests from Renewing or Rebinding states, however RFC makes no
exception for those states, and other implementations (dhclient, lwip)
seem to reuse the same xid in such cases, so comply with this behavior.

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