cxgb3: Fix __must_check warning with dev_dbg.

Fix the warning:
drivers/net/cxgb3/cxgb3_main.c: In function ‘offload_open’:
drivers/net/cxgb3/cxgb3_main.c:936: warning: ignoring return value of
 ‘sysfs_create_group’, declared with attribute warn_unused_result

Now the return value is checked; if sysfs_create_group() returns failure,
a warning is printed using dev_dbg, and the code continues as before.  Use
of dev_dbg ensures printk is not needlessly included unless desired for
debugging.
Signed-off-by: Dan Noe <dpn@isomerica.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/net/cxgb3/cxgb3_main.c

@@ -1014,8 +1014,8 @@ static int offload_open(struct net_device *dev)
 		     adapter->port[0]->mtu : 0xffff);
 	init_smt(adapter);
 
-	/* Never mind if the next step fails */
-	sysfs_create_group(&tdev->lldev->dev.kobj, &offload_attr_group);
+	if (sysfs_create_group(&tdev->lldev->dev.kobj, &offload_attr_group))
+		dev_dbg(&dev->dev, "cannot create sysfs group\n");
 
 	/* Call back all registered clients */
 	cxgb3_add_clients(tdev);