ath10k: Fix mutex unlock balance

ath10k_debug_read_target_stats is properly
protected by data_lock (spinlock). Remove
the unwanted mutex_unlock(&ar->conf_mutex)

[ BUG: bad unlock balance detected! ]
-------------------------------------
kworker/u4:0/12459 is trying to release lock
(&ar->conf_mutex) at:
[<c16a170d>] mutex_unlock+0xd/0x10
but there are no more locks to release!

 Call Trace:
  [<c16a170d>] ? mutex_unlock+0xd/0x10
  [<c10b697d>] __lock_release+0x4d/0xe0
  [<f88ca0fc>] ? ath10k_debug_read_target_stats+0xac/0x290

  [<c16a170d>] ? mutex_unlock+0xd/0x10
  [<c10b6a5b>] lock_release+0x4b/0x150
  [<c16a1580>] __mutex_unlock_slowpath+0x70/0x150
  [<f88ca0fc>] ? ath10k_debug_read_target_stats+0xac/0x290

  [<c10b456b>] ? trace_hardirqs_on+0xb/0x10
  [<c16a170d>] mutex_unlock+0xd/0x10
  [<f88ca107>] ath10k_debug_read_target_stats+0xb7/0x290

  [<f88d337a>] ath10k_wmi_event_process+0x3fa/0x6e0

  [<c10b456b>] ? trace_hardirqs_on+0xb/0x10
  [<f88d36e1>] ath10k_wmi_event_work+0x21/0x40
               [ath10k_core]

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