There was a typo in previous commit

Additional changes:

- Made mipmapping operate with unsigned short instead of char
  which allowed to eliminate extra division by 255, so prevision
  should be a bit better now.

- Actually, this is not real unsigned short range, but it's a
  range of 255*255 which is more convenient for mipmapping, so
  made conversion functions private for scaling.c

  Not sure it worth making this functions operate in 65535
  range, for now current behavior seems to be just fine.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53563 954f8c5b-7b00-dc11-b283-0030488c597c