DAP: Detect SWO buffer overflow

[adamgreig]

Currently SWO buffer overflows are not detectable, as the read method has no way to distinguish how many times the DMA index might have wrapped around compared to the last read position. It might be possible to use the DMA half/full transfer complete flags in some fashion to detect overflow. Alternatively it might be possible to use the UART RXNE interrupt pending bit to poll for that in the application loop, but it seems higher overhead and messier.