ath9k: check ah->curchan when updating tx power

When driver fail to reset card ah->curchan value stay NULL. When
later driver try to update tx power it oops by using ah->curchan
(calltrace is shown below).

This problem were reported at various places and for some it was
fixed by making ath9k_hw_chip_reset() do not fail. I have this bug
report on some oldish RHEL kernel with AR9285, however it's hard to
debug where reset fail when kernel OOPS, so I think this patch
should be applied. Hopefully ah->curchan is not used unconditionally
on other places until is initialized on ath9k_config().

ath: phy0: Chip reset failed
ath: phy0: Unable to reset hardware; reset status -22 (freq 2412 MHz)
BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<f8a35585>] ath9k_hw_set_txpowerlimit+0x25/0x80 [ath9k_hw]
Oops: 0000 [#1] SMP
<snip>
Call Trace:
 [<f8aac1aa>] ? ath9k_cmn_update_txpow+0x1a/0x30 [ath9k_common]
 [<f8cf4f4e>] ? ath_complete_reset+0x4e/0x130 [ath9k]
 [<f8cf54d7>] ? ath9k_start+0x127/0x1e0 [ath9k]
 [<f8c2e52f>] ? ieee80211_do_open+0x30f/0x910 [mac80211]
 [<c07bd96d>] ? dev_open+0x8d/0xf0
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

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

@@ -368,7 +368,7 @@ void ath9k_cmn_update_txpow(struct ath_hw *ah, u16 cur_txpow,
 {
 	struct ath_regulatory *reg = ath9k_hw_regulatory(ah);
 
-	if (reg->power_limit != new_txpow)
+	if (ah->curchan && reg->power_limit != new_txpow)
 		ath9k_hw_set_txpowerlimit(ah, new_txpow, false);
 
 	/* read back in case value is clamped */