net: dsa: vsc73xx: Remove dependency on CONFIG_OF

There is no build time dependency on CONFIG_OF, but we do need to make
sure we gate the initialization of the gpio_chip::of_node member with a
proper check on CONFIG_OF_GPIO. This enables the driver to build on
platforms that do not have CONFIG_OF enabled.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>