power: supply: ab8500: Handle invalid IRQ from platform_get_irq_byname()

platform_get_irq_byname() might return -errno which later would be
cast to an unsigned int and used in request_irq().

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>