pinctrl: mcp23s08: Deduplicate IRQ chip filling

In both cases, SPI and I²C, IRQ chip is filled in the same way.
Deduplicate this by moving common part to mcp23s08_probe_one().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200407173849.43628-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>