s32k3xx:Serial overcome race where DMA has not fetched TCD again"

With TCD set to loop, there is a window where the
   DMA has raised Done, but not reloaded the TCD, resetting
   count and clearing Done.

   In this window  imxrt_dmach_getcount could then return 0.
   Resulting in imxrt_dma_nextrx returning RXDMA_BUFFER_SIZE.
   Which is not a valid index in the FIFO.

   Since the count will be set to RXDMA_BUFFER_SIZE. When the DMA
   engine completes the TCD reload. The imxrt_dma_nextrx would
   return 0. Therefore:

    (RXDMA_BUFFER_SIZE - dmaresidual) % RXDMA_BUFFER_SIZE

   accomplishes this.

arch/arm/src/s32k3xx/s32k3xx_serial.c

@@ -2815,7 +2815,7 @@ static int s32k3xx_dma_nextrx(struct s32k3xx_uart_s *priv)
 {
   int dmaresidual = s32k3xx_dmach_getcount(priv->rxdma);
 
-  return RXDMA_BUFFER_SIZE - dmaresidual;
+  return (RXDMA_BUFFER_SIZE - dmaresidual) % RXDMA_BUFFER_SIZE;
 }
 #endif