gpio: brcmstb: check return value of gpiochip_irqchip_add()

Because gpiochip_irqchip_add() may fail, its return value should
be checked.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>