MIPS: BCM47XX: read sprom without prefix if no ieee80211 core

If there is no ieee80211 core on the devices like on the BCM4706 read
out the sprom and the other data without using a prefix.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: http://patchwork.linux-mips.org/patch/4361


Signed-off-by: John Crispin <blogic@openwrt.org>