block: Simplify usb_msd_initfn() test for "can read bdrv key"

The old test assumes that "hotplugged" implies "we have a current
monitor for reading the key".  This is in fact true, but it's not
obviously true.

Aside: if it were false, we could pass a null pointer to
monitor_read_bdrv_key_start(), which would then crash.

The previous commit permits us to check for "we have a current
monitor" directly, so do that.

hw/usb-msd.c

@@ -535,7 +535,7 @@ static int usb_msd_initfn(USBDevice *dev)
     usb_msd_handle_reset(dev);
 
     if (bdrv_key_required(s->conf.dinfo->bdrv)) {
-        if (s->dev.qdev.hotplugged) {
+        if (cur_mon) {
             monitor_read_bdrv_key_start(cur_mon, s->conf.dinfo->bdrv,
                                         usb_msd_password_cb, s);
             s->dev.auto_attach = 0;