thinkpad-acpi: fix CONFIG_THINKPAD_ACPI_HOTKEY_POLL build problem

Fix this problem when CONFIG_THINKPAD_ACPI_HOTKEY_POLL is undefined:

  CHECK   drivers/platform/x86/thinkpad_acpi.c
drivers/platform/x86/thinkpad_acpi.c:1968:21: error: not an lvalue
  CC [M]  drivers/platform/x86/thinkpad_acpi.o
drivers/platform/x86/thinkpad_acpi.c: In function 'tpacpi_hotkey_driver_mask_set':
drivers/platform/x86/thinkpad_acpi.c:1968: error: lvalue required as left operand of assignment

Reported-by: Noah Dain <noahdain@gmail.com>
Reported-by: Audrius Kazukauskas <audrius@neutrino.lt>
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>