platform/x86: surface3-wmi: fix uninitialized symbol

The patch 3dda3b37: "platform/x86: Add custom surface3 platform
device for controlling LID" from Nov 25, 2016, leads to the following
static checker warning:

        drivers/platform/x86/surface3-wmi.c:168 s3_wmi_check_platform_device()
        error: uninitialized symbol 'ts_adev'.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>