serial: imx: drop useless member from driver data

The wait queue was only initialized and then checked if it contains
active jobs but a job is never added. The last real user was removed
with commit 9d297239 ("serial: imx-serial - update UART IMX driver
to use cyclic DMA").

Further there is no need to release the lock for the check if the port
should be woken up, (and IMHO there never was) so drop the unlock/lock
pair in dma_tx_callback(), too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>