ath6kl: Fix kernel panic on continuous driver load/unload

ath6kl: Fix kernel panic on continuous driver load/unload

On continuous loading and unloading of AR6004 ath6kl USB
driver it triggers a panic due to NULL pointer dereference of
'target' pointer.

while true; do sudo modprobe -v ath6kl_core;
sudo modprobe -v ath6kl_usb; sudo modprobe -r usb;
sudo modprobe -r ath6kl_core; done

ar->htc_target can be NULL due to a race condition that can occur
during driver initialization(we do 'ath6kl_hif_power_on' before
initializing 'ar->htc_target' via 'ath6kl_htc_create').
'ath6kl_hif_power_on' assigns 'ath6kl_recv_complete' as
usb_complete_t/callback function for 'usb_fill_bulk_urb'.
Thus the possibility of ar->htc_target being NULL
via ath6kl_recv_complete -> ath6kl_usb_io_comp_work
before even 'ath6kl_htc_create' is finished to initialize
ar->htc_create.

Worth noting is the obvious solution  of doing 'ath6kl_hif_power_on'
later(i.e after we are done with 'ath6kl_htc_create', causes a
h/w bring up failure in AR6003 SDIO, as 'ath6kl_hif_power_on' is a
pre-requisite to get the target version 'ath6kl_bmi_get_target_info'.
So simply check for NULL pointer for 'ar->htc_target' and bail out.

[23614.518282] BUG: unable to handle kernel NULL pointer dereference at
00000904
[23614.518463] IP: [<c012e7a6>] __ticket_spin_trylock+0x6/0x30
[23614.518570] *pde = 00000000
[23614.518664] Oops: 0000 [#1] SMP
[23614.518795] Modules linked in: ath6kl_usb(O+) ath6kl_core(O)
[23614.520012] EIP: 0060:[<c012e7a6>] EFLAGS: 00010286 CPU: 0
[23614.520012] EIP is at __ticket_spin_trylock+0x6/0x30
Call Trace:
	[<c03f2a44>] do_raw_spin_trylock+0x14/0x40
	[<c06daa12>] _raw_spin_lock_bh+0x52/0x80
	[<f85464b4>] ? ath6kl_htc_pipe_rx_complete+0x3b4/0x4c0 [ath6kl_core]
	[<f85464b4>] ath6kl_htc_pipe_rx_complete+0x3b4/0x4c0 [ath6kl_core]
	[<c05bc272>] ? skb_dequeue+0x22/0x70
	[<c05bc272>] ? skb_dequeue+0x22/0x70
	[<f855bb32>] ath6kl_core_rx_complete+0x12/0x20 [ath6kl_core]
	[<f848771a>] ath6kl_usb_io_comp_work+0xaa/0xb0 [ath6kl_usb]
	[<c015b863>] process_one_work+0x1a3/0x5e0
	[<c015b7e7>] ? process_one_work+0x127/0x5e0
	[<f8487670>] ? ath6kl_usb_reset_resume+0x30/0x30 [ath6kl_usb]
	[<c015bfde>] worker_thread+0x11e/0x3f0
	Kernel panic - not syncing: Fatal exception in interrupt

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