b43: Add simple firmware watchdog

This adds a simple firmware watchdog for the opensource firmware.
This will check every 15 seconds, if the firmware zeroed out the watchdog
register. The firmware will do this in its eventloop.

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