thermal: rcar_gen3_thermal: Fix init value of IRQCTL register

Fix setting value for IRQCTL register. We are setting the last 6 bits
of (IRQCTL) to be 1 (0x3f), this is only suitable for H3ES1.*, according
to Hardware manual values 1 are "setting prohibited" for Gen3.

Signed-off-by: Hoan Nguyen An <na-hoan@jinso.co.jp>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>