[netdrvr] depca: handle platform_device_add() failure

The following patch fixes a kernel bug in depca_platform_probe().

We don't use a dynamic pointer for pldev->dev.platform_data, so it seems
that the correct way to proceed if platform_device_add(pldev) fails is
to explicitly set the pldev->dev.platform_data pointer to NULL, before
calling the platform_device_put(pldev), or it will be kfree'ed by
platform_device_release().

Signed-off-by: Jeff Garzik <jeff@garzik.org>