net: ag71xx: remove unnecessary MTU reservation

2 bytes of the MTU are reserved for Atheros DSA tag, but DSA core
has already handled that since commit dc0fe7d4.
Remove the unnecessary reservation.

Fixes: d51b6ce4 ("net: ethernet: add ag71xx driver")
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20210218034514.3421-1-dqfext@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/atheros/ag71xx.c

@@ -223,8 +223,6 @@
 #define AG71XX_REG_RX_SM	0x01b0
 #define AG71XX_REG_TX_SM	0x01b4
 
-#define ETH_SWITCH_HEADER_LEN	2
-
 #define AG71XX_DEFAULT_MSG_ENABLE	\
 	(NETIF_MSG_DRV			\
 	| NETIF_MSG_PROBE		\
@@ -933,7 +931,7 @@ static void ag71xx_hw_setup(struct ag71xx *ag)
 
 static unsigned int ag71xx_max_frame_len(unsigned int mtu)
 {
-	return ETH_SWITCH_HEADER_LEN + ETH_HLEN + VLAN_HLEN + mtu + ETH_FCS_LEN;
+	return ETH_HLEN + VLAN_HLEN + mtu + ETH_FCS_LEN;
 }
 
 static void ag71xx_hw_set_macaddr(struct ag71xx *ag, unsigned char *mac)