iio: adc: ina2xx: Do not udelay for several seconds

The conversion time can be up to 16 seconds (8 ms per channel, 2 channels,
1024 times averaging).

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>