net: dsa: Properly propagate errors from dsa_switch_setup_one

While shuffling some code around, dsa_switch_setup_one() was introduced,
and it was modified to return either an error code using ERR_PTR() or a
NULL pointer when running out of memory or failing to setup a switch.

This is a problem for its caler: dsa_switch_setup() which uses IS_ERR()
and expects to find an error code, not a NULL pointer, so we still try
to proceed with dsa_switch_setup() and operate on invalid memory
addresses. This can be easily reproduced by having e.g: the bcm_sf2
driver built-in, but having no such switch, such that drv->setup will
fail.

Fix this by using PTR_ERR() consistently which is both more informative
and avoids for the caller to use IS_ERR_OR_NULL().

Fixes: df197195a5248 ("net: dsa: split dsa_switch_setup into two functions")
Reported-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa.c

@@ -359,7 +359,7 @@ dsa_switch_setup(struct dsa_switch_tree *dst, int index,
 	 */
 	ds = kzalloc(sizeof(*ds) + drv->priv_size, GFP_KERNEL);
 	if (ds == NULL)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	ds->dst = dst;
 	ds->index = index;
@@ -370,7 +370,7 @@ dsa_switch_setup(struct dsa_switch_tree *dst, int index,
 
 	ret = dsa_switch_setup_one(ds, parent);
 	if (ret)
-		return NULL;
+		return ERR_PTR(ret);
 
 	return ds;
 }