Input: synaptics-rmi4 - prevent null pointer dereference in f30

If the platform data has f30_data.disable set, f30 in rmi_f30_config()
might be null. Prevent a kernel oops by checking for non-null f30.
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/rmi4/rmi_f30.c

@@ -170,6 +170,10 @@ static int rmi_f30_config(struct rmi_function *fn)
 				rmi_get_platform_data(fn->rmi_dev);
 	int error;
 
+	/* can happen if f30_data.disable is set */
+	if (!f30)
+		return 0;
+
 	if (pdata->f30_data.trackstick_buttons) {
 		/* Try [re-]establish link to F03. */
 		f30->f03 = rmi_find_function(fn->rmi_dev, 0x03);