serial: 8250_dma: Rearm DMA Rx if more data is pending

When DMA Rx completes, the current behavior is to just exit the DMA
completion handler without future actions. If the transfer is still
on-going, UART will trigger an interrupt and that eventually rearms the
DMA Rx. The extra interrupt round-trip has an inherent latency cost
that increases the risk of FIFO overrun. In such situations, the
latency margin tends to already be less due to FIFO not being empty.

Add check into DMA Rx completion handler to detect if LSR has DR (Data
Ready) still set. DR indicates there will be more characters pending
and DMA Rx can be rearmed right away to handle them.

Cc: Gilles BULOZ <[email protected]>
Signed-off-by: Ilpo Järvinen <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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