thinkpad-acpi: deprecate hotkey_bios_mask

Some analysis of the ACPI DSDTs shows that the HKEY pre-enabled mask
is always 0x80c (FN+F3,FN+F4 and FN+F12), which are the hotkeys that
the second gen of HKEY firmware supported (the first gen didn't report
any hotkeys, the second reported these tree hotkeys but had no mask
support, and the third added mask support).

So, this is probably some sort of backwards compatibility with older
versions of the IBM ThinkVantage suite.  We have no use for that, and
I know of exactly ZERO users of that attribute, anyway.  Start the
process of getting rid of it.

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