ath9k_hw: avoid possible infinite loop in ar9003_get_pll_sqsum_dvc

"ath9k: Fix softlockup in AR9485" with commit id
64bc1239c790e051ff677e023435d770d2ffa174 fixed the reported
issue, yet its better to avoid the possible infinite loop
in ar9003_get_pll_sqsum_dvc by having a timeout as suggested
by ath9k maintainers.
http://www.spinics.net/lists/linux-wireless/msg92126.html.
Based on my testing PLL's locking measurement is done in
~200us (2 iterations).

Cc: stable@vger.kernel.org
Cc: Rolf Offermanns <rolf.offermanns@gmx.net>
Cc: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Cc: Senthil Balasubramanian <senthilb@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/hw.c

@@ -784,13 +784,25 @@ static void ath9k_hw_init_qos(struct ath_hw *ah)
 
 u32 ar9003_get_pll_sqsum_dvc(struct ath_hw *ah)
 {
+	struct ath_common *common = ath9k_hw_common(ah);
+	int i = 0;
+
 	REG_CLR_BIT(ah, PLL3, PLL3_DO_MEAS_MASK);
 	udelay(100);
 	REG_SET_BIT(ah, PLL3, PLL3_DO_MEAS_MASK);
 
-	while ((REG_READ(ah, PLL4) & PLL4_MEAS_DONE) == 0)
+	while ((REG_READ(ah, PLL4) & PLL4_MEAS_DONE) == 0) {
+
 		udelay(100);
 
+		if (WARN_ON_ONCE(i >= 100)) {
+			ath_err(common, "PLL4 meaurement not done\n");
+			break;
+		}
+
+		i++;
+	}
+
 	return (REG_READ(ah, PLL3) & SQSUM_DVC_MASK) >> 3;
 }
 EXPORT_SYMBOL(ar9003_get_pll_sqsum_dvc);