rtc: max77686: Properly handle regmap_irq_get_virq() error code

The regmap_irq_get_virq() can return 0 or -EINVAL in error conditions
but driver checked only for value of 0.

This could lead to a cast of -EINVAL to an unsigned int used as a
interrupt number for devm_request_threaded_irq(). Although this is not
yet fatal (devm_request_threaded_irq() will just fail with -EINVAL) but
might be a misleading when diagnosing errors.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Fixes: 6f1c1e71 ("mfd: max77686: Convert to use regmap_irq")
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>