driver: uart: uart_mcux_lpuart fix issues in async api

1. optimized the logic for buffer usage in async api
2. skip timeout flush when the remaining counts is 0,
   as this will trigger dma_callback to process.
3. remove scatter mode, as we are not using this mode
4. trigger after dma_reload.

Signed-off-by: Hake Huang <hake.huang@oss.nxp.com>
1 file changed