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: 970dc85b ("greybus: timesync: Add timesync core driver")
Cc: <stable@vger.kernel.org> # 4.9.x
Signed-off-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>