net: mvpp2: Use field just assigned in error test

In mvpp2_probe the code attempts to get a value for "gop-port-id" and
assigns it to port->gop_id but it then tests port->id for being equal to
-1. That is an impossible test as port->id is a field of type u8 so
cannot be negative. Change the test to port->gop_id.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>

drivers/net/mvpp2.c

@@ -5353,7 +5353,7 @@ static int mvpp2_probe(struct udevice *dev)
 	} else {
 		port->gop_id = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 					      "gop-port-id", -1);
-		if (port->id == -1) {
+		if (port->gop_id == -1) {
 			dev_err(dev, "missing gop-port-id value\n");
 			return -EINVAL;
 		}