lib: sbi: Remove 's' and 'u' from misa_string() output

Both 's' and 'u' are not treated as ISA extensions since these are
privilege modes so let's remove it from misa_string() output.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>