thermal: rcar_thermal: avoid NULL dereference in absence of IRQ resources

Ensure that the base address used by a call to rcar_thermal_common_write()
may be NULL if the SOC supports interrupts for use with the thermal device
but none are defined in DT as is the case for R-Car H1 (r8a7779). Guard
against this condition to prevent a NULL dereference when the device is
probed.

Tested on:
* R-Mobile APE6 (r8a73a4) / APE6EVM
* R-Car H1 (r8a7779) / Marzen
* R-Car H2 (r8a7790) / Lager
* R-Car M2-W (r8a7791) / Koelsch
* R-Car M2-N (r8a7793) / Gose
* R-Car D3 ES1.0 (r8a77995) / Draak

Fixes: 1969d9dc

 ("thermal: rcar_thermal: add r8a77995 support")
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>