mctp i2c: correct mctp_i2c_header_create result

header_ops.create should return the length of the header,
instead mctp_i2c_head_create() returned 0.
This didn't cause any problem because the MCTP stack accepted
0 as success.

Fixes: f5b8abf9 ("mctp i2c: MCTP I2C binding driver")
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/mctp/mctp-i2c.c

@@ -553,7 +553,7 @@ static int mctp_i2c_header_create(struct sk_buff *skb, struct net_device *dev,
 	hdr->source_slave = ((llsrc << 1) & 0xff) | 0x01;
 	mhdr->ver = 0x01;
 
-	return 0;
+	return sizeof(struct mctp_i2c_hdr);
 }
 
 static int mctp_i2c_tx_thread(void *data)