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

In the Linux kernel, the following vulnerability has been resolved: 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().