ath5k/phy.c: fix negative array index

Author: Adrian Bunk <bunk@kernel.org>

This patch fixes a negative array index spotted by the Coverity checker.

Changes-licensed-under: ISC
Acked-by: NNick Kossifidis <mickflemm@gmail.com>
Signed-off-by: NAdrian Bunk <bunk@kernel.org>
Signed-off-by: NLuis R. Rodriguez <mcgrof@winlab.rutgers.edu>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath5k/phy.c

@@ -1178,6 +1178,9 @@ static int ath5k_hw_rf5112_rfregs(struct ath5k_hw *ah,
 			(channel->center_freq >= 5260 ? 1 :
 			    (channel->center_freq > 4000 ? 0 : -1)));
 
+		if (obdb == -1)
+			return -EINVAL;
+
 		if (!ath5k_hw_rfregs_op(rf, ah->ah_offset[6],
 				ee->ee_ob[ee_mode][obdb], 3, 279, 0, true))
 			return -EINVAL;