thermal: rcar_thermal: Prevent hardware access during system suspend

On r8a7791/koelsch, sometimes the following message is printed during
system suspend:

    rcar_thermal e61f0000.thermal: thermal sensor was broken

This happens if the workqueue runs while the device is already
suspended.  Fix this by using the freezable system workqueue instead,
cfr. commit 51e20d0e ("thermal: Prevent polling from happening
during system suspend").

Fixes: e0a5172e ("thermal: rcar: add interrupt support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>