powerpc/powernv: fix construction of opal PRD messages

We currently have a bug in the PRD code, where the contents of an
incoming message (beyond the header) will be overwritten by the list
item manipulations when adding to to the prd_msg_queue.

This change reorders struct opal_prd_msg_queue_item, so that the
message body doesn't overlap the list_head.

We also clarify the memcpy of the message, as we're copying unnecessary
bytes at the end of the message data.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>