net/dhcpv4: Adjust state names to reflect rfc2131

The dhcpv4 state machine implementation names differ from the names
used by the relevant rfc.  There is no good reason for them to be
different, so rename them.

Change-Id: I837f1bc4788c0ed4d2949b12eb6f5bfeef9a0be8
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>