npu3: Make SALT CMD_REG writable
npu3: Make SALT CMD_REG writable

CMD_REG should be writable, not read-only. Fix this, initializing it
with a default "unset" value (0xffffffff).

Signed-off-by: Reza Arbab <4c896a97e6b998365c2d50a267b0f1ca5f7995de@linux.ibm.com>
Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>

shenki/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,open-power/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,shenki/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,open-power/skiboot