brcm47xx: improve detection of Linksys WRTSL54GS

This closes #13319

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


git-svn-id: svn://svn.openwrt.org/openwrt@37647 3c298f89-4303-0410-b956-a3cf2f4a3e73