prd: clean up prd messaging
prd: clean up prd messaging

Skiboot should only send PRD messages to the host kernel if the platform
has enabled opal-prd. This is done via overwriting a function pointer
when the PRD is enabled, but this can be replaced with a simple if check.

Cc: Ananth N Mavinakayanahalli <7dc108d19a2513f6455353af182b9e4d3b794ec9@linux.vnet.ibm.com>
Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

legoater/skiboot,mikey/skiboot,open-power/skiboot,shenki/skiboot,mikey/skiboot,qemu/skiboot,shenki/skiboot,open-power/skiboot,stewart-ibm/skiboot,qemu/skiboot,open-power/skiboot,stewart-ibm/skiboot,qemu/skiboot,legoater/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,legoater/skiboot,mikey/skiboot,stewart-ibm/skiboot,shenki/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot,qemu/skiboot