Match unsigned types

Fixes build warning in MSVC:
vp9\vp9_dx_iface.c(420) : warning C4018: '<' : signed/unsigned mismatch
vp9\vp9_dx_iface.c(423) : warning C4018: '<' : signed/unsigned mismatch

Change-Id: Ia238071aee94b2f882c1e74c380adc3df506dfb6