ASoC: cx20442: set tty->receiver_room in v253_open

Commit 79901317 ("n_tty: Don't flush buffer when closing ldisc"),
introduced in v3.10, revealed a bug in the cx20442 codec driver
which has never been setting tty->receive_room on line discipline
open as it should from the beginning. Fix it.

Created and tested on Amstrad Delta against Linux-4.7-rc3
Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/cx20442.c

@@ -226,6 +226,7 @@ static int v253_open(struct tty_struct *tty)
 	if (!tty->disc_data)
 		return -ENODEV;
 
+	tty->receive_room = 16;
 	if (tty->ops->write(tty, v253_init, len) != len) {
 		ret = -EIO;
 		goto err;