The RX buffer was previously defined as [RX_BUFFER_SIZE/2][2], which
created (RX_BUFFER_SIZE/2) small 2-byte buffers instead of the two
(RX_BUFFER_SIZE/2)-byte buffers required for DMA ping-pong operation.
Update the buffer definition to [2][RX_BUFFER_SIZE/2] and apply the
fix to both the central and peripheral wired split implementations.
This prevents potential DMA RX data corruption when using async UART.
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