staging:iio:adis16209: Set self_test_no_autoclear flag

The ADIS16201 does not automatically clear the self test flag bit the self
test has been, so clear it manually. Otherwise we'll see a offset caused by
the self-test bias on the output values during normal operation.

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