driver core/platform_device_add_resources: set resource to NULL if !res

This makes the res = NULL case more consistant to the res != NULL case
as now both overwrite pdev->resource.

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>