net: wireless: ath9k: avoid possible NULL pointer dereference

Code in ath9k_hw_set_clockrate function indicates that ah->curchan
(and thus chan local variable) may be NULL.  If that is indeed the
case, IS_CHAN_HT40(chan) check has to be performed only in branch
where chan is not NULL.  Moving the code under already existing
if condition fixes this issue.
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -146,10 +146,9 @@ static void ath9k_hw_set_clockrate(struct ath_hw *ah)
 	else
 		clockrate = ATH9K_CLOCK_RATE_5GHZ_OFDM;
 
+	if (chan) {
 		if (IS_CHAN_HT40(chan))
 			clockrate *= 2;
-
-	if (ah->curchan) {
 		if (IS_CHAN_HALF_RATE(chan))
 			clockrate /= 2;
 		if (IS_CHAN_QUARTER_RATE(chan))