ACPI: thinkpad-acpi: cleanup hotkey_notify and HKEY log messages

Use a generic message on hotkey_notify to log unknown and unhandled events,
and cleanup hotkey_notify a little.

Also, document event 0x5010 (brightness changed notification) and do not
log it as an unknown event (even if we do not use it for anything right
now).

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