store quant_shift as an unsigned char

in encodframe.c, quant_shift is set to 0 or 1 in vp8cx_invert_quant

only use 8 bits to store this, instead of 16. will allow saving an
xmm register in an updated version of the regular quantize

Change-Id: Ie88c47fe2aff5af0283dab1147fb2791e4b12f90