core: fix bridge port sysfs directory determination after f5507633 (platform: bridging and bonding options)

The device is not a slave if it *doesn't* have a master.  Code
previously returned an error if the slave did have a master, which
is wrong.

src/platform/nm-linux-platform.c

@@ -1879,7 +1879,7 @@ slave_category (NMPlatform *platform, int slave)
 {
 	int master = link_get_master (platform, slave);
 
-	if (master) {
+	if (master <= 0) {
 		platform->error = NM_PLATFORM_ERROR_NOT_SLAVE;
 		return NULL;
 	}