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>


git-svn-id: svn://svn.openwrt.org/openwrt@37802 3c298f89-4303-0410-b956-a3cf2f4a3e73