platform/x86: thinkpad_acpi: Provide hotkey_poll_stop_sync() dummy

platform/x86: thinkpad_acpi: Provide hotkey_poll_stop_sync() dummy

Provide a hotkey_poll_stop_sync() dummy implementation when
CONFIG_THINKPAD_ACPI_HOTKEY_POLL, so that the #ifdef-ery around
hotkey_poll_stop_sync() can be removed from hotkey_exit().

Tested-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20240424122834.19801-3-hdegoede@redhat.com