drivers: thermal: tsens: Pass around struct tsens_sensor as a constant

All the sensor data is initialised at init time. Lock it down by passing
it to functions as a constant.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/740f9254484c08d65869df578628eb523c0049ff.1584015867.git.amit.kucheria@linaro.org