b43: Fix noise calculation WARN_ON

This removes a WARN_ON that is responsible for the following koops:
http://www.kerneloops.org/searchweek.php?search=b43_generate_noise_sample



The comment in the patch describes why it's safe to simply remove
the check.

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