thinkpad_acpi.c: Fix sparse warning (make undeclared var static)

Fix the following sparse warning:

thinkpad_acpi.c:3459:11: warning: symbol 'adaptive_keyboard_modes' was not declared. Should it be static?

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>