Fix two bugs: - Erroneous shift of 2 in a bytes to bits conversion. - Use reverse subtract rather than negate for value that is subsequently used as operand #2 in a shift operation. Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Reviewed-by: Liming Gao <liming.gao@intel.com>