surfacepro3_button: Add a warning when switching to tablet mode

Microsoft Surface Book has a tablet mode button. Print another message
once on this event instead of repeating "Unknown event...".

Unfortunately, proper support involves the _DSM method, which is not a
discoverable interface. Just print a warning for now.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>