"qc" in vp10_token_state is used to save quantized coefficients, this
commit changes the type from short to tran_low_t to properly reflect
the value range for highbitdepth build.

This fixes an out-of-range bug when optimize_b is used in highbitdepth
build.

Change-Id: I914c6fd3d3f4b9d061f9ed7cc5f08a883ab59dcd