thinkpad-acpi: remove uneeded tp_features.hotkey tests in hotkey_exit

hotkey_exit() is only called if hotkey_init() finished sucessfully, or
by direct calls inside hotkey_init().  The tp_features.hotkey test is
always true, and just adds to the confusion, remove it.  Also, avoid
calling hotkey_mask_set() when it won't do anything useful.

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