brcm47xx: fix detection of Asus RT-N12B1 and Netgear WNR3500L

This closes #14367 and #14328.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38656 3c298f89-4303-0410-b956-a3cf2f4a3e73