pinctrl: mcp23s08: irq mapping is already done

i2c-core and spi-core already assign the irq, so we
can drop the additional call from the mcp driver.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>