internal/cpu: fix style nit in variable name

Consistent with similar change of style in the crypto repository:
http://golang.org/cl/43511

Change-Id: Ib158c52a2649dcbbe9eb92f2bdb9d289e0dcc7bf
Reviewed-on: https://go-review.googlesource.com/53474Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/cpu/cpu_x86.go

@@ -15,9 +15,9 @@ func cpuid(eaxArg, ecxArg uint32) (eax, ebx, ecx, edx uint32)
 func xgetbv() (eax, edx uint32)
 
 func init() {
-	maxId, _, _, _ := cpuid(0, 0)
+	maxID, _, _, _ := cpuid(0, 0)
 
-	if maxId < 1 {
+	if maxID < 1 {
 		return
 	}
 
@@ -43,7 +43,7 @@ func init() {
 
 	X86.HasAVX = isSet(28, ecx1) && osSupportsAVX
 
-	if maxId < 7 {
+	if maxID < 7 {
 		return
 	}