staging:iio: Request threaded-only IRQs with IRQF_ONESHOT

Since commit 1c6c6952 ("genirq: Reject bogus threaded irq requests") threaded
IRQs without a primary handler need to be requested with IRQF_ONESHOT, otherwise
the request will fail. This patch adds the IRQF_ONESHOT to IIO drivers where it
is missing.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>