ath9k_htc: fix handling return value of ath9k_hw_calibrate

The function can return negative values in case of error.
Its result should be then tested for such case.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/assign_signed_to_unsigned.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2046107

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

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

@@ -834,7 +834,7 @@ void ath9k_htc_ani_work(struct work_struct *work)
 		if (longcal || shortcal)
 			common->ani.caldone =
 				ath9k_hw_calibrate(ah, ah->curchan,
-						   ah->rxchainmask, longcal);
+						ah->rxchainmask, longcal) > 0;
 
 		ath9k_htc_ps_restore(priv);
 	}