ath9k: Fix a crash in 2 WIRE btcoex chipsets

Generic timers for BTCOEX functionality is applicable
only for 3 WIRE BTCOEX (and MCI) chipsets.
Hence btcoex->no_stomp_timer is allocated only 3 WIRE
btcoex chipsets and in all the other cases its NULL.
Make sure we stop the generic timer only if
'btcoex->hw_timer_enabled' is true(only if its up and
running)

Fixes the following crash

	[68757.020454] BUG: unable to handle kernel NULL pointer dereference at 0000000c
	[68757.020916] IP: [<f9b055c3>] ath9k_hw_gen_timer_stop+0x13/0x80 [ath9k_hw]
	[68757.021251] *pde = 00000000
	[68757.024384] EIP: 0060:[<f9b055c3>] EFLAGS: 00010082 CPU: 0
	[68757.024384] EIP is at ath9k_hw_gen_timer_stop+0x13/0x80 [ath9k_hw]
	[68757.024384] EAX: d32d0000 EBX: d32d0000 ECX: 00000000 EDX: 00000000
	[68757.024384] ESI: e67c24c0 EDI: 00000296 EBP: e137be2c ESP: e137be20
	[68757.024384]  DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
	[68757.024384] CR0: 8005003b CR2: 0000000c CR3: 00b99000 CR4: 000407d0
	[68757.024384] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
	[68757.024384] DR6: ffff0ff0 DR7: 00000400
	[68757.024384] Process kworker/u:2 (pid: 8917, ti=e137a000 task=ea7a6860 task.ti=e137a000)
	[68757.024384] Stack:
	[68757.024384]  c06c4676 d32d0000 e67c24c0 e137be38 f81c9590 e67c1ca0 e137be40 f81c95d9
	[68757.024384]  e137be64 f81cd1c5 00000246 00000002 d32d0000 e67c05e0 e67c1ca0 e67c05e0
	[68757.024384]  00000000 e137beac f81cdfa0 e137be84 00000246 00000246 e67c1ca0 e67c1ca0
	[68757.024384] Call Trace:
	[68757.024384]  [<c06c4676>] ? _raw_spin_lock_irqsave+0x86/0xa0
	[68757.024384]  [<f81c9590>] ath9k_gen_timer_stop+0x10/0x40 [ath9k]
	[68757.024384]  [<f81c95d9>] ath9k_btcoex_stop_gen_timer+0x19/0x20 [ath9k]
	[68757.024384]  [<f81cd1c5>] ath9k_ps_restore+0x85/0x110 [ath9k]
	[68757.024384]  [<f81cdfa0>] ath9k_config+0x220/0x520 [ath9k]
	[68757.024384]  [<f81cd47d>] ? ath9k_flush+0x15d/0x1b0 [ath9k]
	[68757.024384]  [<f85c7ca5>] ieee80211_hw_config+0x135/0x2c0 [mac80211]
	[68757.024384]  [<f860e3c8>] ieee80211_dynamic_ps_enable_work+0x198/0x5f0 [mac80211]

Cc: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Cc: Bala Shanmugam <bkamatch@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>