ath10k: properly return err from start()

If recovery failed ath10k returned 0 (success) and
mac80211 continued to call other driver callbacks.
This caused null dereference.

This is how the failure looked like:

ath10k: ctl_resp never came in (-110)
ath10k: failed to connect to HTC: -110
ath10k: could not init core (-110)
BUG: unable to handle kernel NULL pointer dereference at           (null)
IP: [<ffffffffa0b355c1>] ath10k_ce_send+0x1d/0x15d [ath10k_pci]
PGD 0
Oops: 0000 [#1] PREEMPT SMP
Modules linked in: ath10k_pci ath10k_core ath5k ath9k ath9k_common ath9k_hw ath mac80211 cfg80211 nf_nat_ipv4 ]
CPU: 1 PID: 36 Comm: kworker/1:1 Tainted: G        WC   3.13.0-rc8-wl-ath+ #8
Hardware name: To be filled by O.E.M. To be filled by O.E.M./HURONRIVER, BIOS 4.6.5 05/02/2012
Workqueue: events ieee80211_restart_work [mac80211]
task: ffff880215b521c0 ti: ffff880215e18000 task.ti: ffff880215e18000
RIP: 0010:[<ffffffffa0b355c1>]  [<ffffffffa0b355c1>] ath10k_ce_send+0x1d/0x15d [ath10k_pci]
RSP: 0018:ffff880215e19af8  EFLAGS: 00010292
RAX: ffff880215e19b10 RBX: 0000000000000000 RCX: 0000000000000018
RDX: 00000000d9ccf800 RSI: ffff8800c965ad00 RDI: 0000000000000000
RBP: ffff880215e19b58 R08: 0000000000000002 R09: 0000000000000000
R10: ffffffff812e1a23 R11: 0000000000000292 R12: 0000000000000018
R13: 0000000000000000 R14: 0000000000000002 R15: ffff88021562d700
FS:  0000000000000000(0000) GS:ffff88021fa80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000001a0d000 CR4: 00000000000407e0
Stack:
 d9ccf8000d47df40 ffffffffa0b367a0 ffff880215e19b10 0000000000000010
 ffff880215e19b68 ffff880215e19b28 0000000000000018 ffff8800c965ad00
 0000000000000018 0000000000000000 0000000000000002 ffff88021562d700
Call Trace:
 [<ffffffffa0b3251d>] ath10k_pci_hif_send_head+0xa7/0xcb [ath10k_pci]
 [<ffffffffa0b16cbe>] ath10k_htc_send+0x23d/0x2d0 [ath10k_core]
 [<ffffffffa0b1a169>] ath10k_wmi_cmd_send_nowait+0x5d/0x85 [ath10k_core]
 [<ffffffffa0b1aaef>] ath10k_wmi_cmd_send+0x62/0x115 [ath10k_core]
 [<ffffffff814e8abd>] ? __netdev_alloc_skb+0x4b/0x9b
 [<ffffffffa0b1c438>] ath10k_wmi_vdev_set_param+0x91/0xa3 [ath10k_core]
 [<ffffffffa0b0e0d5>] ath10k_mac_set_rts+0x3e/0x40 [ath10k_core]
 [<ffffffffa0b0e1d0>] ath10k_set_frag_threshold+0x5e/0x9c [ath10k_core]
 [<ffffffffa09d60eb>] ieee80211_reconfig+0x12a/0x7b3 [mac80211]
 [<ffffffff815a8069>] ? mutex_unlock+0x9/0xb
 [<ffffffffa09b3a40>] ieee80211_restart_work+0x5e/0x68 [mac80211]
 [<ffffffff810c01d0>] process_one_work+0x1d7/0x2fc
 [<ffffffff810c0166>] ? process_one_work+0x16d/0x2fc
 [<ffffffff810c06c8>] worker_thread+0x12e/0x1fb
 [<ffffffff810c059a>] ? rescuer_thread+0x27b/0x27b
 [<ffffffff810c5aee>] kthread+0xb5/0xbd
 [<ffffffff815a9220>] ? _raw_spin_unlock_irq+0x28/0x42
 [<ffffffff810c5a39>] ? __kthread_parkme+0x5c/0x5c
 [<ffffffff815ae04c>] ret_from_fork+0x7c/0xb0
 [<ffffffff810c5a39>] ? __kthread_parkme+0x5c/0x5c
Code: df ff d0 48 83 c4 18 5b 41 5c 41 5d 5d c3 55 48 89 e5 41 57 41 56 45 89 c6 41 55 41 54 41 89 cc 53 48 89
RIP  [<ffffffffa0b355c1>] ath10k_ce_send+0x1d/0x15d [ath10k_pci]
 RSP <ffff880215e19af8>
CR2: 0000000000000000

Reported-By: Ben Greear <greearb@candelatech.com>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>