of_net.h: Provide empty functions if OF_NET is not configured

of_get_mac_address() and of_get_phy_mode() are only provided if OF_NET
is configured. While most callers check for the define, not all do, and those
who do require #ifdef around the code. For those who don't, the missing check
can result in errors such as

arch/powerpc/sysdev/tsi108_dev.c:107:3: error: implicit declaration of
	function 'of_get_mac_address' [-Werror=implicit-function-declaration]
arch/powerpc/sysdev/mv64x60_dev.c:253:2: error: implicit declaration of
	function 'of_get_mac_address' [-Werror=implicit-function-declaration]

Provide empty functions if OF_NET is not configured.
This is safe because all callers do check the return values.

Cc: David Daney <david.daney@cavium.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>