iio: adc: ina2xx: Align timestamp with conversion ready flag

As the timestamp is no longer (ab-)used to measure the function run time,
it can be taken at the correct time, i.e. when the conversion has finished.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>