b43: Reject new firmware early

(not in mainline, as it is not applicable.)

We must reject new incompatible firmware early to avoid
running into strange transmission failures.

The current development tree supports newer firmware revisions.
These revisions cause strange failures on the stable 2.6.24 kernel.
Add a check to avoid confusing users a lot.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>