net: switchdev: add SET_SWITCHDEV_OPS helper

Add a helper to allow switchdev ops to be set if NET_SWITCHDEV is configured
and do nothing otherwise. This allows for slightly cleaner code which
uses switchdev but does not select NET_SWITCHDEV.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>