pinctrl: iproc: allow for error from platform_get_irq()

platform_get_irq() can return an error code. Allow for this when getting
the irq.

Fixes: 6f265e5d

 ("pinctrl: bcm-iproc: Pass irqchip when adding gpiochip")
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20191003000310.17099-2-chris.packham@alliedtelesis.co.nz
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>