b43legacy: add me as maintainer and fix URLs

As b43legacy is going to be orphaned, add me as a maintainer. Fix URLs for
the related website and fix my e-mail address in MAINTAINERS file.

Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Cc: Larry Finger <larry.finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>