iio: dac: adi-axi-dac: modify stream enable

Change suggested from the AXI HDL team, modify the function
axi_dac_data_stream_enable() to check for interface busy, to avoid
possible issues when starting the stream.

Fixes: e61d7178429a ("iio: dac: adi-axi-dac: extend features")
Reviewed-by: Nuno Sa <[e-mail]>
Signed-off-by: Angelo Dureghello <[e-mail]>
Link: https://patch.msgid.link/202[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/dac/adi-axi-dac.c

···
 static int axi_dac_data_stream_enable(struct iio_backend *back)
 {
 	struct axi_dac_state *st = iio_backend_get_priv(back);
+	int ret, val;
+
+	ret = regmap_read_poll_timeout(st->regmap,
+				AXI_DAC_UI_STATUS_REG, val,
+				FIELD_GET(AXI_DAC_UI_STATUS_IF_BUSY, val) == 0,
+				10, 100 * KILO);
+	if (ret)
+		return ret;
 
 	return regmap_set_bits(st->regmap, AXI_DAC_CUSTOM_CTRL_REG,
 			       AXI_DAC_CUSTOM_CTRL_STREAM_ENABLE);