vt82c686: fix coverity warning about out-of-bounds write

Refactor superio_ioport_writeb to fix the out of bounds write warning.

In addition, fix two typos: s/chage/change/
Signed-off-by: Nzhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/isa/vt82c686.c

@@ -50,13 +50,13 @@ typedef struct VT82C686BState {
 static void superio_ioport_writeb(void *opaque, hwaddr addr, uint64_t data,
                                   unsigned size)
 {
-    int can_write;
     SuperIOConfig *superio_conf = opaque;
 
     DPRINTF("superio_ioport_writeb  address 0x%x  val 0x%x\n", addr, data);
     if (addr == 0x3f0) {
         superio_conf->index = data & 0xff;
     } else {
+        bool can_write = true;
         /* 0x3f1 */
         switch (superio_conf->index) {
         case 0x00 ... 0xdf:
@@ -68,31 +68,28 @@ static void superio_ioport_writeb(void *opaque, hwaddr addr, uint64_t data,
         case 0xf7:
         case 0xf9 ... 0xfb:
         case 0xfd ... 0xff:
-            can_write = 0;
+            can_write = false;
             break;
-        default:
-            can_write = 1;
-
-            if (can_write) {
-                switch (superio_conf->index) {
         case 0xe7:
             if ((data & 0xff) != 0xfe) {
-                        DPRINTF("chage uart 1 base. unsupported yet\n");
+                DPRINTF("change uart 1 base. unsupported yet\n");
+                can_write = false;
             }
             break;
         case 0xe8:
             if ((data & 0xff) != 0xbe) {
-                        DPRINTF("chage uart 2 base. unsupported yet\n");
+                DPRINTF("change uart 2 base. unsupported yet\n");
+                can_write = false;
             }
             break;
-
         default:
-                    superio_conf->config[superio_conf->index] = data & 0xff;
-                }
-            }
+            break;
+
         }
+        if (can_write) {
             superio_conf->config[superio_conf->index] = data & 0xff;
         }
+    }
 }
 
 static uint64_t superio_ioport_readb(void *opaque, hwaddr addr, unsigned size)