Do not zero memory in umass_detach

The detach function is called very often, for example from
get_capacity function. We don't want to loose any pointers
here, so disable detaching for umass driver.

Submitted by:  Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Sponsored by:  Juniper Networks Inc.
Differential Revision: https://reviews.freebsd.org/D4141

sys/boot/usb/storage/umass_common.c

@@ -82,6 +82,9 @@ umass_attach(device_t dev)
 static int
 umass_detach(device_t dev)
 {
+
+#ifdef USB_DEBUG
 	memset(&umass_uaa, 0, sizeof(umass_uaa));
+#endif
 	return (0);
 }