brcm47xx: add detection code for Huawei E970

This is based on a patch by Mathias Adam.

Signed-off-by: Mathias Adam <m.adam--openwrt@adamis.de>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38001 3c298f89-4303-0410-b956-a3cf2f4a3e73