staging: brcm80211: deleted struct dot11_bcn_prb

Code cleanup. This struct did nothing useful in the code. Instances of this
struct and the code that read them were removed as well.

Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>