thinkpad-acpi: fix poll thread auto-start

The driver was not starting the NVRAM polling thread if the input
device was bound immediately after registration.

This fixes:
http://bugzilla.kernel.org/show_bug.cgi?id=15118



Reported-by: Florian Zumbiehl <florz@florz.de>
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: stable@kernel.org