riscv: vector: add a comment when calling riscv_setup_vsize()

riscv: vector: add a comment when calling riscv_setup_vsize()

The function would fail when it detects the calling hart's vlen doesn't
match the first one's. The boot hart is the first hart calling this
function during riscv_fill_hwcap, so it is impossible to fail here. Add
a comment about this behavior.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240510-zve-detection-v5-1-0711bdd26c12@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>