MAINTAINERS: wifi: mark b43 as orphan

There's no maintainer for b43 so mark it as orphan.

Signed-off-by: Kalle Valo <kvalo@kernel.org>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230724104547.3061709-9-kvalo@kernel.org