irq_reg_stride in struct regmap_irq_chip is often 0, but that
actually means to use the default stride of 1. The effective
stride is stored in struct regmap_irq_chip_data->irq_reg_stride
and will get the corrected default value.
The default ->get_irq_reg() callback was using the stride from
the chip definition, which is wrong; fix it to use the effective
stride from the chip data instead.