b43legacy: fix typo in ifdef comment

Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>