Bugfix: ICMPv4 TYPE match using incorrect enum.

Fix ICMPv4 TYPE match field to use OFPXMT_OFB_ICMPV4_TYPE instead of
OFPXMT_OFB_ICMPV4_CODE.
Merge pull request #6 from bertvl-netro/icmpv4_type_fix

Bugfix: ICMPv4 TYPE match using incorrect enum.