MIPS: BCM47XX: Fix some very confused types and data corruption

Fix nvram_read_alpha2 copying too many bytes over the ssb_sprom
structure. Also fix the arguments of the read_macaddr, although the code
was technically not wrong before due to an extra dereference.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6211/