MLK-15093 tty: serial: imx: enable bit TDMAEN in each DMA transfer

MLK-15093 tty: serial: imx: enable bit TDMAEN in each DMA transfer

In below case:
write() -> flush() -> write() -> flush() ...

.imx_flush_buffer() _MAY_ clear UCR1_TDMAEN bit if the callback is not
comming or DMA transfer is not completed, to ensure DMA trigger is enabled
for the new DMA prep_sg, enable the UCR1_TDMAEN bit in .dma_tx_work().

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: David Wolfe <david.wolfe@nxp.com>