iio:adis: Add support for manual self-test flag clear

Some variants of the devices from the ADIS family don't auto-clear the
self-test bit after the self-test has completed. Instead we have to
manually clear. Add support for this to the ADIS library.

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