staging: brcm80211: removed superfluous forward struct declarations

Code cleanup.

Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Reported-by: Julian Calaby <julian.calaby@gmail.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>