serial/mxs-auart: only wake up tty layer once

There is no need to call uart_write_wakeup after each character send.
Once at the end of the write sequence is enough.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>