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

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 <nuno.sa@analog.com>
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250114-wip-bl-ad3552r-axi-v0-iio-testing-carlos-v4-3-979402e33545@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>