iio:adc:ad7766: Use new IRQF_NO_AUTOEN to reduce boilerplate

As iio_poll_trigger() is safe against spurious interrupts when the
trigger is not enabled, this is not a fix despite looking like
a race.  It is nice to simplify the code however so the interrupt
is never enabled in the first place.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Barry Song <song.bao.hua@hisilicon.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210402184544.488862-2-jic23@kernel.org