platform/x86: thinkpad_acpi: take mutex for hotkey_mask_{set,get}

platform/x86: thinkpad_acpi: take mutex for hotkey_mask_{set,get}

hotkey_mask_set() and hotkey_mask_get() expect hotkey_mutex to be held.
While it should not matter during initialization a following patch will
enable lockdep for hotkey_mutex which would produce warnings here.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20230704-thinkpad_acpi-lockdep-v1-1-60129548a738@weissschuh.net
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>