net: pch_gbe: Fix rx descriptor buffer addresses

The loop to set up buffer addresses in rx descriptors always operated on
descriptor 0, rather than on each descriptor sequentially. Fix this in
order to setup correct buffer addresses for each descriptor.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>