staging:iio:ad7606: Run trigger handler only once per trigger event

Currently the ad7606 driver installs the same function for the hard-irq and
threaded trigger handlers. This was introduced in commit 1caf7cb4
("staging:iio:adc:ad7606 Convert to new channel registration method Update
Add missing call to iio_trigger_notify_done() Set pollfunc top and bottom
half handler"). Unfortunately the commit message does not mention why this
was done and Michael does not remember either.

Since the trigger handler function is idempotent (set a GPIO to 1) running
it twice does not do any harm, but is simply not necessary either. So set
the threaded trigger handler for the driver to NULL.

While we are at it also remove the function description comment that does
no say anything that can't be derived from the function name itself.

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