iio: adc: xilinx-ams: Fix single channel switching sequence

Some of the AMS channels need to be read by switching into single-channel
mode from the normal polling sequence. There was a logic issue in this
switching code that could cause the first read of these channels to read
back as zero.

It appears that the sequencer should be set back to default mode before
changing the channel selection, and the channel should be set before
switching the sequencer back into single-channel mode.

Also, write 1 to the EOC bit in the status register to clear it before
waiting for it to become set, so that we actually wait for a new
conversion to complete, and don't proceed based on a previous conversion
completing.

Fixes: d5c70627 ("iio: adc: Add Xilinx AMS driver")
Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20220127173450.3684318-5-robert.hancock@calian.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>