Keep using the interface append_oxm_match_arp_op() and add an exception in oxm-helper.c

Please refer to #117

This is a better solution that keep using the append_oxm_match_arp_op() interface such that existing application don't need any modification.

However, we can not use APPEND_OXM_MATCH macro directly in oxm-helper.c
An exception is added to that file just like how we did to vlan_vid field (near line 480-488)