MAINTAINERS: wifi: brcm80211: cleanup entry

There has been some discussion about what is expected from
a maintainer and so a cleanup seems to be in order. A dedicated
mailing list has been created to discuss brcm80211 specific
development issues. Keeping the status as Supported although
help in maintaining this driver is welcomed.

Cc: brcm80211@lists.linux.dev
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240126105724.384063-1-arend.vanspriel@broadcom.com