RISC-V: Use the minor version mask while computing sbi version

As per the SBI specification, minor version is encoded in the
lower 24 bits only. Make sure that the SBI version is computed
with the appropriate mask.

Currently, there is no minor version in use. Thus, it doesn't
change anything functionality but it is good to be compliant with
the specification.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20240420151741.962500-8-atishp@rivosinc.com
Signed-off-by: Anup Patel <anup@brainfault.org>