riscv: Fix the __riscv_copy_vec_words_unaligned implementation

riscv: Fix the __riscv_copy_vec_words_unaligned implementation

Correct the VEC_S macro definition to fix the implementation
of vector words copy in the case of unalignment in RISC-V.

Fixes: e7c9d66e313b ("RISC-V: Report vector unaligned access speed hwprobe")
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Tingbo Liao <tingbo.liao@starfivetech.com>
Link: https://lore.kernel.org/r/20250228090801.8334-1-tingbo.liao@starfivetech.com
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>