platform/x86: acer-wmi: Silence "unsupported" message a bit

This driver prints that "Unsupported machine..." message on every boot on
ThinkPad X1 Carbon laptops (and I assume a number of other systems), which
causes graphical boots to "glitch" a bit and is rather annoying ...

Make it a pr_debug instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>