ACPI: thinkpad-acpi: warn once about weird hotkey masks

thinkpad-acpi knows for a while now how to best program the hotkeys by
default, and always enable them by default.  Unfortunately, this
information has not filtered down everywhere it needs to, yet.  Notably,
old ibm-acpi documentation and most "thinkpad setup guides" will have wrong
information on this area.

Warn the local admin once whenever any of the following patterns are met:

1. Attempts to set hotkey mask to 0xffff (artifact from docs and config
   for the old ibm-acpi driver and behaviour).  This mask makes no
   real-world sense;

2. Attempts to set hotkey mask to 0xffffffff, which means the user is
   trying to just have "everything work" without even reading the
   documentation, or that we need to get a bug report, because there
   is a new thinkpad out there with new exciting hot keys :-)

3. Attempts to set hotkey mask to 0xffffff, which is almost never the
   correct way to set up volume and brightness event reporting (and with
   the current state-of-the-art, it is known to never be right way to do
   it).

The driver will perform any and all requested operations, though,
regardless of any warnings.  I hope these warnings can be removed one or
two years from now.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>