of_net: fix of_get_mac_address retval if compiled without CONFIG_OF

of_get_mac_address prior to commit d01f449c ("of_net: add NVMEM
support to of_get_mac_address") could return only valid pointer or NULL,
after this change it could return only valid pointer or ERR_PTR encoded
error value, but I've forget to change the return value of
of_get_mac_address in case where the kernel is compiled without
CONFIG_OF, so I'm doing so now.

Cc: Mirko Lindner <mlindner@marvell.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Fixes: d01f449c ("of_net: add NVMEM support to of_get_mac_address")
Reported-by: NOctavio Alvarez <octallk1@alvarezp.org>
Signed-off-by: NPetr Štetiar <ynezz@true.cz>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/of_net.h

@@ -22,7 +22,7 @@ static inline int of_get_phy_mode(struct device_node *np)
 
 static inline const void *of_get_mac_address(struct device_node *np)
 {
-	return NULL;
+	return ERR_PTR(-ENODEV);
 }
 
 static inline struct net_device *of_find_net_device_by_node(struct device_node *np)