iio: ti_am335x_adc: Fix wrong samples received on 1st read

Previously we tried to read data form ADC even before ADC sequencer
finished sampling. This led to wrong samples.
We now wait on ADC status register idle bit to be set.

Signed-off-by: Patil, Rachna <rachna@ti.com>
Signed-off-by: Zubair Lutfullah <zubair.lutfullah@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>