b43: Add debugfs firmware debugging knob

This adds a firmware debugging knob to debugfs.
With this knob it's possible to enable advanced runtime firmware
checks.
For now it only implements one sanity check for the mac-suspend.
In future there'll probably be more.
If CONFIG_B43_DEBUG is disabled, these checks will collapse to nothing.

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