powerpc/powernv: Use struct opal_prd_msg in more places

Use the newly added struct opal_prd_msg in some other functions that
operate on opal_prd messages, rather than using other types.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230821142820.497107-2-mpe@ellerman.id.au