iio: adc: cpcap-adc: Fix missing IRQF_ONESHOT as only threaded handler.

Coccinelle noticed:
CHECK   drivers/iio/adc/cpcap-adc.c
drivers/iio/adc/cpcap-adc.c:1009:9-34: ERROR: Threaded IRQ with no primary handler requested without IRQF_ONESHOT

As far as I can see this is a simple case of it should be specified
but isn't.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Tony Lindgren <tony@atomide.com>