platform/x86: silead_dmi - do not treat all devices as i2c_clients

I2C bus has both i2c clients and adapter devices, so we must be careful in
notifier code and verify that we are actually dealing with an i2c client
before using it as such.

Fixes: cef9dd85

 ("platform/x86: add support for devices with Silead...")
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
[andy: simplified silead_ts_dmi_add_props() change]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>