internal/chacha20: fix minor naming issue

This corrects a compile error due to a mismatch between function
description and the assembler code that first appeared when running
TryBots on the change to vendor this code. This fixes that problem
and adds and minor change based on a comment in CL 195959.

Change-Id: I42cbd3f75e281913f12f74850deaedf14eb21989
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/205659
Reviewed-by: Filippo Valsorda <filippo@golang.org>

internal/chacha20/asm_ppc64le.s

@@ -72,13 +72,13 @@ DATA consts<>+0x90(SB)/8, $0x0000000100000000
 DATA consts<>+0x98(SB)/8, $0x0000000300000002
 GLOBL consts<>(SB), RODATA, $0xa0
 
-//func chaCha20_ctr32_vsx(out, inp []byte, len int, key *[32]byte, counter *[16]byte)
+//func chaCha20_ctr32_vsx(out, inp *byte, len int, key *[8]uint32, counter *uint32)
 TEXT ·chaCha20_ctr32_vsx(SB),NOSPLIT,$64-40
 	MOVD out+0(FP), OUT
 	MOVD inp+8(FP), INP
 	MOVD len+16(FP), LEN
 	MOVD key+24(FP), KEY
-	MOVD cnt+32(FP), CNT
+	MOVD counter+32(FP), CNT
 
 	// Addressing for constants
 	MOVD $consts<>+0x00(SB), CONSTBASE

internal/chacha20/chacha_ppc64le.go

@@ -10,9 +10,10 @@ import (
 	"encoding/binary"
 )
 
-var haveAsm = true
-
-const bufSize = 256
+const (
+	bufSize = 256
+	haveAsm = true
+)
 
 //go:noescape
 func chaCha20_ctr32_vsx(out, inp *byte, len int, key *[8]uint32, counter *uint32)