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: Octavio Alvarez <octallk1@alvarezp.org>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>