Use MV_FP_SIZE based constant instead of 3.

Change-Id: I90ef3b49b499c2ac9c24797467cb4eb194fdf23b
diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index 7d69d78..402dc06 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -217,7 +217,7 @@
     nmv_component *const comp_ctx = &ctx->comps[i];
     for (j = 0; j < CLASS0_SIZE; ++j)
       update_mv_probs(comp_ctx->class0_fp[j], MV_FP_SIZE - 1, r);
-    update_mv_probs(comp_ctx->fp, 3, r);
+    update_mv_probs(comp_ctx->fp, MV_FP_SIZE - 1, r);
   }
 
   if (allow_hp) {