ACPI: thinkpad-acpi: silence _sta warning

When both CONFIG_THINKPAD_ACPI_DOCK and CONFIG_THINKPAD_ACPI_BAY are
undefined, _sta is not used and that causes a gcc warning.  Fix it
(and I think this is a regression, I am pretty sure I fixed this once
before, sorry about that).

Issue reported by: Pritt Laes.

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