tty: serial: fsl_lpuart: use the sg count from dma_map_sg

The dmaengine_prep_slave_sg needs to use sg count returned
by dma_map_sg, not use sport->dma_tx_nents, because the return
value of dma_map_sg is not always same with "nents".

When enabling iommu for lpuart + edma, iommu framework may concatenate
two sgs into one.

Fixes: 6250cc30 ("tty: serial: fsl_lpuart: Use scatter/gather DMA for Tx")
Cc: <stable@vger.kernel.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/1572932977-17866-1-git-send-email-peng.fan@nxp.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/fsl_lpuart.c

@@ -437,8 +437,8 @@ static void lpuart_dma_tx(struct lpuart_port *sport)
 	}
 
 	sport->dma_tx_desc = dmaengine_prep_slave_sg(sport->dma_tx_chan, sgl,
-					sport->dma_tx_nents,
-					DMA_MEM_TO_DEV, DMA_PREP_INTERRUPT);
+					ret, DMA_MEM_TO_DEV,
+					DMA_PREP_INTERRUPT);
 	if (!sport->dma_tx_desc) {
 		dma_unmap_sg(dev, sgl, sport->dma_tx_nents, DMA_TO_DEVICE);
 		dev_err(dev, "Cannot prepare TX slave DMA!\n");