net/dhcpv4: Fix the behaviour of ciaddr.

Correct the logic that determines when CIADDR is set and when it is
left as all zeros. RFC2131 requires that CIADDR is only set when the
client is in specific states.

Change-Id: I93a45e355fbef8d11dba5ac194570b87c594656e
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>