rtlwifi: rtl8192c: Fix W=1 warning

When this driver is built with "make W=1", the following warning occurs:

drivers/net/wireless/rtlwifi/rtl8192c/dm_common.c:907:4: warning: comparison is always false due to limited range of data type [-Wtype-limits]
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rtlwifi/rtl8192c/dm_common.c

@@ -669,7 +669,8 @@ static void rtl92c_dm_txpower_tracking_callback_thermalmeter(struct ieee80211_hw
 	u8 thermalvalue, delta, delta_lck, delta_iqk;
 	long ele_a, ele_d, temp_cck, val_x, value32;
 	long val_y, ele_c = 0;
-	u8 ofdm_index[2], cck_index = 0, ofdm_index_old[2], cck_index_old = 0;
+	u8 ofdm_index[2], ofdm_index_old[2], cck_index_old = 0;
+	s8 cck_index = 0;
 	int i;
 	bool is2t = IS_92C_SERIAL(rtlhal->version);
 	s8 txpwr_level[2] = {0, 0};