mfd: sprd-sc27xx-spi: Fix-up bogus IRQ register offset and mask setting

mfd: sprd-sc27xx-spi: Fix-up bogus IRQ register offset and mask setting

'i / pdata->num_irqs' always equates to 0 and 'BIT(i % pdata->num_irqs)'
always ends up being BIT(i) here, so make that clearer in the code.  If
the code base needs to support more than 32 IRQs in the future, this will
have to be reworked, but lets just keep it simple for as long as we can.

This fixes the following W=1 warning:

 drivers/mfd/sprd-sc27xx-spi.c:255 sprd_pmic_probe() debug: sval_binop_unsigned: divide by zero

Cc: Orson Zhai <orsonzhai@gmail.com>
Cc: Chunyan Zhang <zhang.lyra@gmail.com>
Suggested-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>