irqchip/renesas-rzg2l: Convert to irq_data_get_irq_chip_data()

Use the existing irq_data_get_irq_chip_data() helper instead of
open-coding the same operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/8e47cc6400e5a82c854c855948d2665a3a3197e3.1695819391.git.geert+renesas@glider.be