tty: serial: imx: 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".

Fixes: b4cdc8f6 ("serial: imx: add DMA support for imx6q")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/1573108875-26530-1-git-send-email-peng.fan@nxp.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>