rtc: da9063: 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>
Tested-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Link: https://lore.kernel.org/r/20191004150510.6278-1-krzk@kernel.org


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>