virtio: don't fail on !of_device_is_compatible

A recent change checking of_device_is_compatible on probe broke some
powerpc/pseries setups. Apparently there virtio devices do not have a
"compatible" property - they are matched by PCI vendor/device ids.

Let's just skip of_node setup but proceed with initialization like we
did previously.

Fixes: 694a1116 ("virtio: Bind virtio device to device-tree node")
Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>