driver core/platform_device_add_data: set platform_data to NULL if !data

This makes the data = NULL case more consistent to the data != NULL case.
The functional change is that now

	platform_device_add_data(somepdev, NULL, somesize)

sets pdev->dev.platform_data to NULL instead of not touching it.

Reviewed-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>