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>