thinkpad-acpi: hotkey poll fixes

Fix some locking, avoid exiting the kthread before kthread_stop() is
called on it, and clean up the hotkey poll routines a little bit.

Also, restore bits in the firmware mask after hotkey_source_mask is
changed.  Without this, we leave events disabled...

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