b43legacy: fix shared IRQ race condition

Fix an IRQ race condition in b43legacy. If we call
b43legacy_wireless_core_stop(), it will set the status of the device to
INITIALIZED and the IRQ handler won't care any longer about IRQs, thus the
kernel will disable the IRQ if it's shared (unless we boot it with the
'irqpoll' option). So we must disable IRQs before changing the device
status.

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