net/dhcpv4: Fix event/state mismatch

Fix long standing issue where a dhcpv4 message type is compared
against a dhcpv4 state machine state name rather than a message type.
The issue probably arizes due to the similarity in names between
messages and states.  By accident, the relevant message types and
states happen to share the same numbers, hence the implementation
works, but is ill defined.

Change-Id: I5c028de4336ff42f6696e28b3492c932c58b5a05
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>