b43legacy: fix PIO crash

Fix the crash reported below, which seems to happen on bcm4306 rev. 2 devices
only while using PIO:

Oops: 0000 [#1] PREEMPT
Modules linked in: b43(F) rfkill(F) led_class(F) input_polldev(F) arc4 b43legacy mac80211 cfg80211 i915 drm snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device ohci1394 ieee1394 ssb pcmcia snd_intel8x0m ehci_hcd uhci_hcd evdev

Pid: 0, comm: swapper Tainted: GF	(2.6.24st3 #2)
EIP: 0060:[<f90f667b>] EFLAGS: 00010002 CPU: 0
EIP is at b43legacy_pio_handle_txstatus+0xbb/0x210 [b43legacy]
EAX: 0000049b EBX: f11f8044 ECX: 00000001 EDX: 00000000
ESI: f1ff8000 EDI: 00000000 EBP: f11f8040 ESP: c04f4ef4
 DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068
Process swapper (pid: 0, ti=c04f4000 task=c0488300 task.ti=c04b8000)
Stack: f90f2788 c05009f0 c0500900 000010f7 f1053823 c04f4f24 dfb8e800 00000003
       f1368000 00000007 00000296 f90f1975 00001000 010c0800 01000000 00000007
       f90f6391 f11f8000 00000082 c04f4f4a 00000000 00004fd0 10f70000 8c061000
Call Trace:
 [<f90f2788>] b43legacy_debugfs_log_txstat+0x48/0xb0 [b43legacy]
 [<f90f1975>] b43legacy_handle_hwtxstatus+0x75/0x80 [b43legacy]
 [<f90f6391>] b43legacy_pio_rx+0x201/0x280 [b43legacy]
 [<f90e4fa3>] b43legacy_interrupt_tasklet+0x2e3/0x870 [b43legacy]
 [<c0123567>] tasklet_action+0x27/0x60
 [<c01237b4>] __do_softirq+0x54/0xb0
 [<c010686b>] do_softirq+0x7b/0xe0
 [<c01457c0>] handle_level_irq+0x0/0x110
 [<c01457c0>] handle_level_irq+0x0/0x110
 [<c0123758>] irq_exit+0x38/0x40
 [<c0106953>] do_IRQ+0x83/0xd0
 [<c011812f>] __update_rq_clock+0x4f/0x180
 [<c0104b4f>] common_interrupt+0x23/0x28
 [<c011007b>] wakeup_code+0x7b/0xde
 [<c02b1039>] acpi_processor_idle+0x24a/0x3c9
 [<c01025c7>] cpu_idle+0x47/0x80
 [<c04b9ad5>] start_kernel+0x205/0x290
 [<c04b9360>] unknown_bootoption+0x0/0x1f0
 =======================
Code: 0f 00 00 81 fb ff 00 00 00 0f 87 36 01 00 00 8d 04 db 85 ff 8d 6c c6 40 8d 5d 04 0f 85 ef 00 00 00 fe 4e 0e 0f b7 46 0c 8b 53 04 <8b> 4a 50 29 c8 83 e8 52 66 89 46 0c 8b 54 24 14 80 7a 0b 00 74
EIP: [<f90f667b>] b43legacy_pio_handle_txstatus+0xbb/0x210 [b43legacy] SS:ESP 0068:c04f4ef4
Kernel panic - not syncing: Fatal exception in interrupt

Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>