RISC-V: hwprobe: Explicity check for -1 in vdso init

RISC-V: hwprobe: Explicity check for -1 in vdso init

id_bitsmash is unsigned. We need to explicitly check for -1, rather
than use > 0.

Fixes: aa5af0aa90ba ("RISC-V: Add hwprobe vDSO function and data")
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Evan Green <evan@rivosinc.com>
Link: https://lore.kernel.org/r/20230426141333.10063-3-ajones@ventanamicro.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>