modules/admin-full: fix support for broadcom-wl modes

This adds N modes support for proprietary Broadcom WiFi driver and adds a/b/g modes detection.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Merge pull request #221 from openwrt-es/luci-next

modules/admin-full: fix support for broadcom-wl modes