tipc: set skb->protocol in eth_media packet transmission

The skb->protocol field is used by packet classifiers and for AF_PACKET
cooked format, TIPC needs to set it properly.

Fixes packet classification and ethertype of 0x0000 in cooked captures:

Out 20:c9:d0:43:12:d9 ethertype Unknown (0x0000), length 56:
	0x0000:  5b50 0028 0000 30d4 0100 1000 0100 1001  [P.(..0.........
	0x0010:  0000 03e8 0000 0001 20c9 d043 12d9 0000  ...........C....
	0x0020:  0000 0000 0000 0000                      ........

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>