net/dhcpv4: Refactor dhcpv4 message type representation.

Change the message type representation from integer to enum, this will
help prevent re-occurrence of recent issues where states and events
are interchanged by making it more likely the compiler will see a type
mismatch.

Change-Id: Ia235afda428a9e5dfbd933e02beeae468b4c84a2
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>