net/dhcpv4: Use unique XID on each request.

Rather than reuse XID's on retransmissions, always use a unique XID.

Either behaviour is permitted by rfc2131.  Debugging the dhcp client
in the presence of multiple dhcp servers with significant packet loss
in the network is much easier if we don't unnecessarily reuse request
identifiers.

Change-Id: I5c82cbdbf3dfc0ef88cae036aa863946844c144f
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>