net: mvneta: fix error message when MTU too large for XDP

The error message says that "Jumbo frames are not supported on XDP", but
the code checks for mtu > MVNETA_MAX_RX_BUF_SIZE, not mtu > 1500.

Fix this error message.
Signed-off-by: Marek Behún <kabel@kernel.org>
Fixes: 0db51da7 ("net: mvneta: add basic XDP support")
Cc: Lorenzo Bianconi <lorenzo@kernel.org>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Link: https://lore.kernel.org/r/20210105172333.21613-1-kabel@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/marvell/mvneta.c

@@ -4432,7 +4432,7 @@ static int mvneta_xdp_setup(struct net_device *dev, struct bpf_prog *prog,
 	struct bpf_prog *old_prog;
 
 	if (prog && dev->mtu > MVNETA_MAX_RX_BUF_SIZE) {
-		NL_SET_ERR_MSG_MOD(extack, "Jumbo frames not supported on XDP");
+		NL_SET_ERR_MSG_MOD(extack, "MTU too large for XDP");
 		return -EOPNOTSUPP;
 	}