smm: Switch from %edx to %ebx

%edx gets clobbered before the c handler is invoked. This is just a
cleanup cl to make the next cl look clean.

BUG=b:80539294
TEST=verified SMI still works on grunt.

Change-Id: I21bf41ed4fdeaaa8737c883f202a39cb57c2b517
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-on: https://review.coreboot.org/27228
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/cpu/x86/smm/smm_stub.S b/src/cpu/x86/smm/smm_stub.S
index 0c05032..ba66db9 100644
--- a/src/cpu/x86/smm/smm_stub.S
+++ b/src/cpu/x86/smm/smm_stub.S
@@ -132,9 +132,9 @@
 1:
 	movl	stack_size, %eax
 	mul	%ecx /* %eax(stack_size) * %ecx(cpu) = %eax(offset) */
-	movl	stack_top, %edx
-	subl	%eax, %edx /* global_stack_top - offset = stack_top */
-	mov	%edx, %esp
+	movl	stack_top, %ebx
+	subl	%eax, %ebx /* global_stack_top - offset = stack_top */
+	mov	%ebx, %esp
 
 	/* Create stack frame by pushing a NULL stack base pointer */
 	pushl	$0x0