arm: vp9itxfm16: Use the right lane size

This makes the code slightly clearer, but doesn't make any functional
difference.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/arm/vp9itxfm_16bpp_neon.S

@@ -1082,8 +1082,8 @@ A       and             r7,  sp,  #15
 .ifc \txfm1\()_\txfm2,idct_idct
         b               3f
 1:
-        vmov.i16        q14, #0
-        vmov.i16        q15, #0
+        vmov.i32        q14, #0
+        vmov.i32        q15, #0
 2:
         subs            r1,  r1,  #1
         @ Unroll for 2 lines