thinkpad-acpi: make drivers/misc/thinkpad_acpi:fan_mutex static

This patch makes the needlessly global fan_mutex static.

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