iio:adc:ad_sigma_delta: Use IRQF_NO_AUTOEN rather than request and disable

These devices are not able to mask the signal used as a data ready
interrupt.  As such they previously requested the irq then immediately
disabled it.  Now we can avoid the potential of a spurious interrupt
by avoiding the irq being auto enabled in the first place.

I'm not sure how this code could have been called with the irq already
disabled, so I believe the conditional would always have been true and
have removed it.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Alexandru Ardelean <ardeleanalex@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210402184544.488862-8-jic23@kernel.org