Fixes the following W=1 kernel build warning(s):

 drivers/of/of_net.c:104: warning: Function parameter or member 'np' not described in 'of_get_mac_address'
 drivers/of/of_net.c:104: warning: expecting prototype for mac(). Prototype was for of_get_mac_address() instead

Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: netdev@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210318104036.3175910-9-lee.jones@linaro.org