arm: vp9itxfm: Rename a macro parameter to fit better

Since the same parameter is used for both input and output,
the name inout is more fitting.

This matches the naming used below in the dmbutterfly macro.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/arm/vp9itxfm_neon.S

@@ -125,16 +125,16 @@ endconst
         vmlal.s16       \out4, \in4, \coef1
 .endm
 
-@ in1 = (in1 * coef1 - in2 * coef2 + (1 << 13)) >> 14
-@ in2 = (in1 * coef2 + in2 * coef1 + (1 << 13)) >> 14
-@ in are 2 d registers, tmp are 2 q registers
-.macro mbutterfly in1, in2, coef1, coef2, tmp1, tmp2, neg=0
-        mbutterfly_l    \tmp1, \tmp2, \in1, \in2, \coef1, \coef2
+@ inout1 = (inout1 * coef1 - inout2 * coef2 + (1 << 13)) >> 14
+@ inout2 = (inout1 * coef2 + inout2 * coef1 + (1 << 13)) >> 14
+@ inout are 2 d registers, tmp are 2 q registers
+.macro mbutterfly inout1, inout2, coef1, coef2, tmp1, tmp2, neg=0
+        mbutterfly_l    \tmp1, \tmp2, \inout1, \inout2, \coef1, \coef2
 .if \neg > 0
         vneg.s32        \tmp2, \tmp2
 .endif
-        vrshrn.s32      \in1, \tmp1,  #14
-        vrshrn.s32      \in2, \tmp2,  #14
+        vrshrn.s32      \inout1, \tmp1,  #14
+        vrshrn.s32      \inout2, \tmp2,  #14
 .endm
 
 @ inout1,inout2 = (inout1,inout2 * coef1 - inout3,inout4 * coef2 + (1 << 13)) >> 14