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

The current code only wakes up the processes when the circle
buffer has less data then the WAKEUP_CHARS.

But sometimes, the circle buffer may has data more then the WAKEUP_CHARS,
in such case, the processes will hang.

This patch makes it always wakes up the processes in the TX callback.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>