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

drivers/misc/ti-st/st_core.c

@@ -52,13 +52,12 @@ static void remove_channel_from_table(struct st_data_s *st_gdata,
  */
 int st_get_uart_wr_room(struct st_data_s *st_gdata)
 {
-	struct tty_struct *tty;
 	if (unlikely(st_gdata == NULL || st_gdata->tty == NULL)) {
 		pr_err("tty unavailable to perform write");
 		return -1;
 	}
-	tty = st_gdata->tty;
-	return tty->ops->write_room(tty);
+
+	return tty_write_room(st_gdata->tty);
 }
 
 /*