net/faraday: Add missing include of of.h

Breaking the include loop netdevice.h, dsa.h, devlink.h broke this
driver, it depends on includes brought in by these headers. Adding
linux/of.h fixes it.

Fixes: ed0e39e97d34 ("net: break include loop netdevice.h, dsa.h, devlink.h")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/faraday/ftgmac100.c

@@ -28,6 +28,7 @@
 #include <linux/io.h>
 #include <linux/module.h>
 #include <linux/netdevice.h>
+#include <linux/of.h>
 #include <linux/phy.h>
 #include <linux/platform_device.h>
 #include <net/ip.h>