ASoC: cx20442: tty_ldisc_ops::write_wakeup is optional

TTY layer does nothing if tty_ldisc_ops::write_wakeup is NULL, so there
is no need to implement an empty one in cx20442. Drop it.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210722115141.516-1-jslaby@suse.cz


Signed-off-by: Mark Brown <broonie@kernel.org>