staging: greybus: timesync: validate platform state callback

When tearingdown timesync, and not in arche platform, the state platform
callback is not initialized. That will trigger the following NULL
dereferencing.
CallTrace:

? gb_timesync_platform_unlock_bus+0x11/0x20 [greybus]
gb_timesync_teardown+0x85/0xc0 [greybus]
gb_timesync_svc_remove+0xab/0x190 [greybus]
gb_svc_del+0x29/0x110 [greybus]
gb_hd_del+0x14/0x20 [greybus]
ap_disconnect+0x24/0x60 [gb_es2]
usb_unbind_interface+0x7a/0x2c0
__device_release_driver+0x96/0x150
device_release_driver+0x1e/0x30
bus_remove_device+0xe7/0x130
device_del+0x116/0x230
usb_disable_device+0x97/0x1f0
usb_disconnect+0x80/0x260
hub_event+0x5ca/0x10e0
process_one_work+0x126/0x3b0
worker_thread+0x55/0x4c0
? process_one_work+0x3b0/0x3b0
kthread+0xc4/0xe0
? kthread_park+0xb0/0xb0
ret_from_fork+0x22/0x30

So, fix that by adding checks before use the callback.

Fixes: 970dc85bd95d ("greybus: timesync: Add timesync core driver")
Cc: <[e-mail]> # 4.9.x
Signed-off-by: Rui Miguel Silva <[e-mail]>
Reviewed-by: Viresh Kumar <[e-mail]>
Reviewed-by: Johan Hovold <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/greybus/timesync_platform.c

···
 
 int gb_timesync_platform_lock_bus(struct gb_timesync_svc *pdata)
 {
+	if (!arche_platform_change_state_cb)
+		return 0;
+
 	return arche_platform_change_state_cb(ARCHE_PLATFORM_STATE_TIME_SYNC,
 					      pdata);
 }
 
 void gb_timesync_platform_unlock_bus(void)
 {
+	if (!arche_platform_change_state_cb)
+		return;
+
 	arche_platform_change_state_cb(ARCHE_PLATFORM_STATE_ACTIVE, NULL);
 }