iio: adc: ad_sigma_delta: Fix a race condition

iio: adc: ad_sigma_delta: Fix a race condition

The ad_sigma_delta driver helper uses irq_disable_nosync(). With that
one it is possible that the irq handler still runs after the
irq_disable_nosync() function call returns. Also to properly synchronize
irq disabling in the different threads proper locking is needed and
because it's unclear if the irq handler's irq_disable_nosync() call
comes first or the one in the enabler's error path, all code locations
that disable the irq must check for .irq_dis first to ensure there is
exactly one disable call per enable call.

So add a spinlock to the struct ad_sigma_delta and use it to synchronize
irq enabling and disabling. Also only act in the irq handler if the irq
is still enabled.

Fixes: af3008485ea0 ("iio:adc: Add common code for ADI Sigma Delta devices")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/9e6def47e2e773e0e15b7a2c29d22629b53d91b1.1733504533.git.u.kleine-koenig@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>