iio: adc: ad7923: Fix buffer overflow for tx_buf and ring_xfer

iio: adc: ad7923: Fix buffer overflow for tx_buf and ring_xfer

The AD7923 was updated to support devices with 8 channels, but the size
of tx_buf and ring_xfer was not increased accordingly, leading to a
potential buffer overflow in ad7923_update_scan_mode().

Fixes: 851644a60d20 ("iio: adc: ad7923: Add support for the ad7908/ad7918/ad7928")
Cc: stable@vger.kernel.org
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Zicheng Qu <quzicheng@huawei.com>
Link: https://patch.msgid.link/20241029134637.2261336-1-quzicheng@huawei.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>