usb-storage: fix possible memory leak and missing error message

When scsi_bus_legacy_add_drive() return NULL, meanwhile err will
be not NULL, which will casue memory leak and missing error message.

Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
