b43: Add panic reason code that doesn't trigger restart

Add a firmware panic reason code that doesn't trigger a restart.
This is useful for firmware debugging and avoiding endless
restart loops. We can use FWPANIC_DIE to halt the firmware at a
well defined point.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>