staging: ti-soc-thermal: fix device removal

While removing, the device needs to unregister
the sensor from thermal framework. Before
calling the call back the driver needs to check
if the call back is registered. This patch
fix the check by checking the right callback.

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>