stacks: There is no need to disable NMI if it is already disabled

Don't write to the cmos index port on a mode switch if NMI is already
disabled.  This reduces the number of outb() calls.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/stacks.c b/src/stacks.c
index f4d15ce..2fe1bfb 100644
--- a/src/stacks.c
+++ b/src/stacks.c
@@ -66,8 +66,10 @@
 
     // Backup cmos index register and disable nmi
     u8 cmosindex = inb(PORT_CMOS_INDEX);
-    outb(cmosindex | NMI_DISABLE_BIT, PORT_CMOS_INDEX);
-    inb(PORT_CMOS_DATA);
+    if (!(cmosindex & NMI_DISABLE_BIT)) {
+        outb(cmosindex | NMI_DISABLE_BIT, PORT_CMOS_INDEX);
+        inb(PORT_CMOS_DATA);
+    }
     SET_LOW(Call16Data.cmosindex, cmosindex);
 
     SET_LOW(Call16Data.method, method);
@@ -103,8 +105,11 @@
     }
 
     // Restore cmos index register
-    outb(GET_LOW(Call16Data.cmosindex), PORT_CMOS_INDEX);
-    inb(PORT_CMOS_DATA);
+    u8 cmosindex = GET_LOW(Call16Data.cmosindex);
+    if (!(cmosindex & NMI_DISABLE_BIT)) {
+        outb(cmosindex, PORT_CMOS_INDEX);
+        inb(PORT_CMOS_DATA);
+    }
     return method;
 }