iio: cf_axi_dds: remove st->enable handling in buffer stream

Nothing is being done between when this is set to false & back to true.
Also, the cf_axi_dds_start_sync() function is called with 1/true which is
only needed when the PCORE is at an older version (than major 8).

This change would also reset the `enable` state to true, and probably cause
some out-sync configuration/stuff.

Removing this is the last piece to make the cf_axi_dds_state struct opaque.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>

drivers/iio/frequency/cf_axi_dds_buffer_stream.c

@@ -57,11 +57,9 @@ static int dds_buffer_state_set(struct iio_dev *indio_dev, bool state)
 		return cf_axi_dds_datasel(st, -1, DATA_SEL_DDS);
 
 	cf_axi_dds_stop(st);
-	st->enable = false;
 
 	dds_write(st, ADI_REG_VDMA_STATUS, ADI_VDMA_OVF | ADI_VDMA_UNF);
 
-	st->enable = true;
 	cf_axi_dds_start_sync(st, 1);
 
 	return 0;