ath10k: fix fetching channel during potential radar detection

Fetch target operating channel during potential radar detection when
the interface is just brought up, but no channel is assigned from
userspace. In this scenario rx_channel may not be having a valid pointer
hence fetch the target operating channel to avoid warnings as below
which can be triggered by the commands with DFS testing over longer run

comamnds:
iw wlan1 set type mesh
ifconfig wlan1 up (valid tgt_oper_chan only)
iw wlan1 cac trigger freq 5260 HT20 (valid rx_channel, tgt_oper_chan)
iw wlan1 cac trigger freq 5280 HT20
iw wlan1 cac trigger freq 5300 HT20

Once the CAC expires, current channel context will be removed and
we are only left with the fallback option of using 'target operating
channel'

Firmware and driver log:
ath: phy1: DFS: radar found on freq=5300: id=1, pri=1125, count=5,
count_false=4
ath: phy1: DFS: radar found on freq=5260: id=5, pri=3151, count=6,
count_false=11
ath: phy1: DFS: radar found on freq=5280: id=1, pri=1351, count=6,
count_false=4
ath: phy1: DFS: radar found on freq=5300: id=1, pri=1125, count=5,
count_false=4
ath10k_pci 0001:01:00.0: failed to derive channel for radar pulse,
treating as radar
ath10k_pci 0001:01:00.0: failed to derive channel for radar pulse,
treating as radar

Call trace:

WARNING: CPU: 1 PID: 2145 at
backports-20161201-3.14.77-9ab3068/net/wireless/chan.c:265
cfg80211_set_dfs_state+0x3c/0x88 [cfg80211]()

 Workqueue: phy1 ieee80211_dfs_radar_detected_work
[mac80211]
[<c0320770>] (warn_slowpath_null) from [<bf79b90c>]
(cfg80211_set_dfs_state+0x3c/0x88 [cfg80211])
[<bf79b90c>] (cfg80211_set_dfs_state [cfg80211]) from
[<bf79697c>] (cfg80211_radar_event+0xc4/0x140 [cfg80211])
[<bf79697c>] (cfg80211_radar_event [cfg80211]) from
[<bf83c058>] (ieee80211_dfs_radar_detected_work+0xa8/0xb4 [mac80211])
[<bf83c058>] (ieee80211_dfs_radar_detected_work
[mac80211]) from [<c0339518>] (process_one_work+0x298/0x4a4)

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>