net: of: fix stub of_net helpers for CONFIG_NET=n

Moving the of_net code from drivers/of/ to net/core means we
no longer stub out the helpers when networking is disabled,
which leads to a randconfig build failure with at least one
ARM platform that calls this from non-networking code:

arm-linux-gnueabi-ld: arch/arm/mach-mvebu/kirkwood.o: in function `kirkwood_dt_eth_fixup':
kirkwood.c:(.init.text+0x54): undefined reference to `of_get_mac_address'

Restore the way this worked before by changing that #ifdef
check back to testing for both CONFIG_OF and CONFIG_NET.

Fixes: e330fb14

 ("of: net: move of_net under net/")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20211014090055.2058949-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>