pinctrl: mcp23s08: fix irq and irqchip setup order

Since 'commit 02e389e6 ("pinctrl: mcp23s08: fix irq setup order")' the
irq request isn't the last devm_* allocation. Without a deeper look at
the irq and testing this isn't a good solution. Since this driver relies
on the devm mechanism, requesting a interrupt should be the last thing
to avoid memory corruptions during unbinding.

'Commit 02e389e6 ("pinctrl: mcp23s08: fix irq setup order")' fixed the
order for the interrupt-controller use case only. The
mcp23s08_irq_setup() must be split into two to fix it for the
interrupt-controller use case and to register the irq at last. So the
irq will be freed first during unbind.

Cc: stable@vger.kernel.org
Cc: Jan Kundrát <jan.kundrat@cesnet.cz>
Cc: Dmitry Mastykin <mastichi@gmail.com>
Cc: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Fixes: 82039d24 ("pinctrl: mcp23s08: add pinconf support")
Fixes: 02e389e6 ("pinctrl: mcp23s08: fix irq setup order")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Tested-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>