thermal: rcar_gen3_thermal: Update calculation formula of IRQTEMP

Update the formula to calculate CTEMP:
Currently, the CTEMP is average of val1 (is calculated by
formula 1) and val2 (is calculated by formula 2). But,
as description in HWM (chapter 10A.3.1.1 Setting of Normal Mode)

If (STEMP < Tj_T) CTEMP value should be val1.
If (STEMP > Tj_T) CTEMP value should be val2.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>