[brcm47xx]: Fix reading sprom from nvram without a prefix

[brcm47xx]: Fix reading sprom from nvram without a prefix

The bcma based SoCs with a ieee80211 core on the SoC and an other
connected via PCIe or USB store the sprom for the SoC with a sb/1/
prefix. The SoC with just one wifi core do not use prefixes. The
BCM4706 do not use a prefix for the SoC part at all, because the prefix
is the path to the ieee80211 core and there is non on the BCM4706.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@33597 3c298f89-4303-0410-b956-a3cf2f4a3e73