ti-st: use tty_write_room

Don't access tty->ops->write_room directly, use tty_write_room helper
instead.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210505091928.22010-22-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>