Fix addressing mistake when splitting MCB

kernel/mcb.asm

@@ -26,6 +26,7 @@ mcb_init:
 mcb_split:
 	call mcb_find
 	dec dx
+	push dx
 	; Calculate size of first block
 	; dx = dx - ds - 1
 	stc
@@ -40,9 +41,10 @@ mcb_split:
 	xchg cl, byte [si+0]
 	; advance DS to begin of first block
 	pop ax
-	mov ds, ax
 	stc
 	sbb dx, ax
+	pop ax
+	mov ds, ax
 	; write MCB data
 	mov byte [si+0], cl
 	mov word [si+1], 0