ether: add EtherType for proprietary Realtek protocols

Add a new EtherType ETH_P_REALTEK to the if_ether.h uapi header. The
EtherType 0x8899 is used in a number of different protocols from Realtek
Semiconductor Corp [1], so no general assumptions should be made when
trying to decode such packets. Observed protocols include:

  0x1 - Realtek Remote Control protocol [2]
  0x2 - Echo protocol [2]
  0x3 - Loop detection protocol [2]
  0x4 - RTL8365MB 4- and 8-byte switch CPU tag protocols [3]
  0x9 - RTL8306 switch CPU tag protocol [4]
  0xA - RTL8366RB switch CPU tag protocol [4]

[1] https://lore.kernel.org/netdev/CACRpkdYQthFgjwVzHyK3DeYUOdcYyWmdjDPG=Rf9B3VrJ12Rzg@mail.gmail.com/
[2] https://www.wireshark.org/lists/ethereal-dev/200409/msg00090.html
[3] https://lore.kernel.org/netdev/20210822193145.1312668-4-alvin@pqrs.dk/
[4] https://lore.kernel.org/netdev/20200708122537.1341307-2-linus.walleij@linaro.org/Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/if_ether.h

@@ -86,6 +86,7 @@
 					 * over Ethernet
 					 */
 #define ETH_P_PAE	0x888E		/* Port Access Entity (IEEE 802.1X) */
+#define ETH_P_REALTEK	0x8899          /* Multiple proprietary protocols */
 #define ETH_P_AOE	0x88A2		/* ATA over Ethernet		*/
 #define ETH_P_8021AD	0x88A8          /* 802.1ad Service VLAN		*/
 #define ETH_P_802_EX1	0x88B5		/* 802.1 Local Experimental 1.  */