net: dsa: tag_rtl4_a: Use existing ETH_P_REALTEK constant

No functional change, uses the existing ETH_P_REALTEK constant already
defined in if_ether.h.

Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20231113165030.2440083-1-florian.fainelli@broadcom.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>