drivers/usb: use lock to protect reqbuf when cdcacm unbind

When CONFIG_CDCACM_DISABLE_TXBUF is enabled, if the USB is
unplugged during serial data transmission, it is possible
that cdcacm will enter the unbind process, and reqbuf will
be released, causing a crash during serial write.

Signed-off-by: yangsong8 <yangsong8@xiaomi.com>

drivers/usbdev/cdcacm.c

@@ -1497,6 +1497,10 @@ static void cdcacm_unbind(FAR struct usbdevclass_driver_s *driver,
        * of them)
        */
 
+#ifdef CONFIG_CDCACM_DISABLE_TXBUF
+      nxmutex_lock(&priv->serdev.xmit.lock);
+#endif
+
       flags = spin_lock_irqsave(&priv->lock);
 
 #ifdef CONFIG_CDCACM_DISABLE_TXBUF
@@ -1524,6 +1528,10 @@ static void cdcacm_unbind(FAR struct usbdevclass_driver_s *driver,
       DEBUGASSERT(priv->nwrq == 0);
       spin_unlock_irqrestore(&priv->lock, flags);
 
+#ifdef CONFIG_CDCACM_DISABLE_TXBUF
+      nxmutex_unlock(&priv->serdev.xmit.lock);
+#endif
+
 #ifdef CONFIG_CDCACM_HAVE_EPINTIN
       /* Free the interrupt IN endpoint */