irqchip/sun6i-r: Use NULL for chip_data

sparse complains about using an integer as a NULL pointer.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220424173952.36591-1-samuel@sholland.org