powerpc/pseries/cmm: Remove unneeded variable "ret"

Remove unneeded variable: "ret".

Signed-off-by: zuoqilin <zuoqilin@yulong.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Just return NOTIFY_OK directly]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210326075619.48-1-zuoqilin1@163.com