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

This reverts commit 2ad28e3efee21a5bbf940c83d1f0395b76bd3efb.

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 2ad28e3 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 <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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