ENGR00321246 tty: serial: imx: fix wakeup fail after suspend for more than 30s

ENGR00321246 tty: serial: imx: fix wakeup fail after suspend for more than 30s

Before DMA finish, we have to disable flow control, otherwise
there have one corner issue like:
Flow control enable, RTS always is high while there have no uart
terminal connect to imx uart, and then user transmit data by the
uart, after some time, TX FIFO is _FULL_, SDMA still don't complete
the current transcation, so hold on. There no SDMA interrupt generate,
the "dma_wait" event cannot be waked up.

(cherry picked from commit a614a7d01705e1db69956913c810b62bc7f0c83e)

Signed-off-by: Fugang Duan <B38611@freescale.com>