staging: omap-thermal: move conv table limits out of sensor data

As we have one conv table per bandgap device and not per sensor,
this patch changes the data structures so that the conv table
min and max values are now part of bandgap_data and not sensor_data.

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