hw/riscv/riscv-iommu: Fix PPN field of Translation-reponse register

hw/riscv/riscv-iommu: Fix PPN field of Translation-reponse register

The original implementation incorrectly performed a bitwise AND
operation between the PPN of iova and PPN Mask, leading to an
incorrect PPN field in Translation-reponse register.

The PPN of iova should be set entirely in the PPN field of
Translation-reponse register.

Also remove the code that was used to clear S field since this
field is already zero.

Signed-off-by: Nutty Liu <liujingqi@lanxincomputing.com>
Reviewed-by: Tomasz Jeznach <tjeznach@rivosinc.com>
Message-ID: <20250605124848.1248-1-liujingqi@lanxincomputing.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>