powerpc/pseries: Remove unused r0 in the hcall tracing code

In the plpar_hcall trace code, currently we use r0
to store the value of r4. But this value is not
used subsequently in the code. Hence remove this unused
save to r0 in plpar_hcall and plpar_hcall9

Suggested-by: Naveen N Rao <naveen@kernel.org>
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230929172337.7906-2-atrajeev@linux.vnet.ibm.com