ath9k: Fix bug in reading debugfs file 'rcstat'

The rate table would not have been chosen before the interface
has been brought up. Reading 'rcstat' in this case would result
in an oops, fix this.
Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath9k/debug.c

@@ -322,6 +322,9 @@ static ssize_t read_file_rcstat(struct file *file, char __user *user_buf,
 {
 	struct ath_softc *sc = file->private_data;
 
+	if (sc->cur_rate_table == NULL)
+		return 0;
+
 	if (conf_is_ht(&sc->hw->conf))
 		return ath_read_file_stat_11n_rc(file, user_buf, count, ppos);
 	else