Revert "serial: imx: always wake up the processes in the TX callback"

This reverts commit 2ad28e3e.

Instead of always wake up write_wait process in TX callback,
TX callback should call imx_dma_tx() again, and let imx_dma_tx
transfer the remaining data in circle buffer.

The issue with commit 2ad28e3e is, in case there is remaining
data in circle buffer, but no process is waiting on write_wait
queue, then as no following uart_write() will be called after
uart_write_wakeup(), thus cause data loss.

Moreover according to Documentation/serial/driver, uart_write_wakeup()
should be called in case the transmit buffer have dropped below
a threshold.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>