libertas : Remove unused variable warning for "old_channel" from cmd.c

Below patch removes the following warning during compilation.

drivers/net/wireless/libertas/cmd.c:826: warning: unused variable 'old_channel'

Signed-off-by : Manish Katiyar <mkatiyar@gmail.com>
Acked-by: NDan Williams <dcbw@redhat.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/libertas/cmd.c

@@ -823,7 +823,9 @@ int lbs_update_channel(struct lbs_private *priv)
 int lbs_set_channel(struct lbs_private *priv, u8 channel)
 {
 	struct cmd_ds_802_11_rf_channel cmd;
+#ifdef DEBUG
 	u8 old_channel = priv->curbssparams.channel;
+#endif
 	int ret = 0;
 
 	lbs_deb_enter(LBS_DEB_CMD);