decoder: h264: fix long-term reference picture marking process.

Fix reference picture marking process with memory_management_control_op
set to 3 and 6, i.e. assign LongTermFrameIdx to a short-term reference
picture, or the current picture.

This fixes decoding of FRExt_MMCO4_Sony_B.

Signed-off-by: Zhong Cong <congx.zhong@intel.com>