Validate, that the definition is in sync with other definitions.
Excerpt from contrib/tcpdump/ethertype.h
'''
#ifndef ETHERTYPE_8021Q
#define ETHERTYPE_8021Q 0x8100
#endif
/* see:
http://en.wikipedia.org/wiki/IEEE_802.1Q
and http://en.wikipedia.org/wiki/QinQ
*/
#ifndef ETHERTYPE_8021Q9100
#define ETHERTYPE_8021Q9100 0x9100
#endif
#ifndef ETHERTYPE_8021Q9200
#define ETHERTYPE_8021Q9200 0x9200
#endif
#ifndef ETHERTYPE_8021QinQ
#define ETHERTYPE_8021QinQ 0x88a8
#endif
'''