broadcom-wl: Add 'N' mode support to broadcom-wl

This is partly based on patches sent by Łukasz Kwestarz (See https://lists.openwrt.org/pipermail/openwrt-devel/2012-March/014356.html

).

Signed-off-by: Nathan Hintz <nlhintz@hotmail.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

SVN-Revision: 37802