s390/ctcm: fix variable dereferenced before check

Found by cppcheck and smatch.
smatch complains about
drivers/s390/net/ctcm_sysfs.c:43 ctcm_buffer_write() warn: variable dereferenced before check 'priv' (see line 42)

Fixes: 3c09e2647b5e ("ctcm: rename READ/WRITE defines to avoid redefinitions")
Reported-by: Colin Ian King <[e-mail]>
Signed-off-by: Alexandra Winter <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/s390/net/ctcm_sysfs.c

···
 	struct ctcm_priv *priv = dev_get_drvdata(dev);
 	int rc;
 
-	ndev = priv->channel[CTCM_READ]->netdev;
-	if (!(priv && priv->channel[CTCM_READ] && ndev)) {
+	if (!(priv && priv->channel[CTCM_READ] &&
+	      priv->channel[CTCM_READ]->netdev)) {
 		CTCM_DBF_TEXT(SETUP, CTC_DBF_ERROR, "bfnondev");
 		return -ENODEV;
 	}
+	ndev = priv->channel[CTCM_READ]->netdev;
 
 	rc = kstrtouint(buf, 0, &bs1);
 	if (rc)