iio: hx711: add delay until DOUT is ready

On a system with parasitic capacitance it turned out that DOUT is not ready
after 100 ns after PD_SCK has raised. A measurement showed almost 1000 ns
until DOUT has reached its correct value.

With this patch its now possible to wait until data is ready.

The wait time should not be higher than the maximum PD_SCK high time which
is corresponding to the datasheet 50000 ns.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>