A temporary fix to mips sub-pel filters

A temporary fix to mips sub-pel filters

There are Jenkins test failures in mips sub-pel filter optimizations.
[ RUN      ] MSA/ConvolveTest.MatchesReferenceSubpixelFilter/5
../libvpx/test/convolve_test.cc:889: Failure
Expected equality of these values:
  lookup(ref, y * kOutputStride + x)
    Which is: 255
  lookup(out, y * kOutputStride + x)
    Which is: 11
mismatch at (1,0), filters (4,0,1)

This relates to the 4-tap kernel added recently. This CL is a temporary
fix, while we investigate the issue.

BUG=webm:1568

Change-Id: If64c552b794425687cca4fbed893d8ccb73c89a5