thinkpad-acpi: don't leave ERR_PTR() pointers around

backlight_device_register returns ERR_PTR() in case of problems, and
the current code would leave that ERR_PTR in ibm_backlight_device.

The current code paths won't touch it in that situation, but that could
change.  Make sure to set ibm_backlight_device to NULL in the error
path.

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