staging: brcm80211: replaced struct dot11_header by struct ieee80211_hdr

Code cleanup. Replaced Broadcom specific structure by its Linux equivalent.

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>