iio: adc: cpcap: fix incorrect validation

Function platform_get_irq_byname() returns a negative error code on
failure, and a zero or positive number on success. However, in function
cpcap_adc_probe(), positive IRQ numbers are also taken as error cases.
Use "if (ddata->irq < 0)" instead of "if (!ddata->irq)" to validate the
return value of platform_get_irq_byname().

Signed-off-by: Pan Bian <bianpan2016@163.com>
Fixes: 25ec2496 ("iio: adc: cpcap: Add minimal support for CPCAP PMIC ADC")
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Tony Lindgren <tony@atomide.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>