brcm47xx: do not use GPIO configuration of WRT54G for every unknown Linksys device

Most of the Linksys devices without an own GPIO configuration where 
detected as WRT54G. On one board this made OpenWrt to boot into 
failsafe every time, because one GPIO configurated as button was not a 
button.

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


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37970 3c298f89-4303-0410-b956-a3cf2f4a3e73