cx20442: tty_ldisc_ops::write_wakeup is optional

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.

Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: alsa-devel@alsa-project.org
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210722115141.516-1-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>