thermal: rcar: change type of ctemp in rcar_thermal_update_temp()

Since the ctemp is used for rcar_thermal_write() in
rcar_thermal_update_temp(), the type of 'ctemp' should be "u32" instead
of "int". This patch also changes type of the helper variables 'old'
and 'new'.

Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>