SERIAL DRIVER WITH DMA DOES NOT DISCARD OOB CHARACTERS

```
Description: If Ctrl-Z or Ctrl-C actions are enabled, the the OOB
character that generates the signal action must not be placed
in the serial driver Rx buffer. This behavior is correct for
the non-DMA case (serial_io.c), but not for the DMA case
(serial_dma.c). In the DMA case, the OOB character is left
in the Rx buffer and will be received as normal Rx data by
the application. It should not work that way.

Perhaps in the DMA case, the OOB characters could be filtered
out later, just before returning the Rx data to the application?
Status: Open
Priority: Low, provided that the application can handle these characters
in the data stream.
```

Read serial_io.c and serial_dma.c, comparing how OOB Ctrl-Z and Ctrl-C characters are handled before data reaches the Rx buffer. Verify that the DMA path excludes signal-generating OOB characters from application-visible Rx data, matching the non-DMA behavior.

Written by the indexing model from the issue text.