fastboot: demote OS X USB error.

We can't create interface iterators for all devices (e.g. the keyboard,
touchbar, etc.), so we get multiple instances of this log spam on every
command.

Demote it to a warning that only shows up on debug builds.

Test: none
Change-Id: I20581b9134c05360d5723b09ffe8de8c4cfd6a3f
diff --git a/fastboot/usb_osx.cpp b/fastboot/usb_osx.cpp
index f597f50..8a3c213 100644
--- a/fastboot/usb_osx.cpp
+++ b/fastboot/usb_osx.cpp
@@ -106,7 +106,7 @@
     kr = (*dev)->CreateInterfaceIterator(dev, &request, &iterator);
 
     if (kr != 0) {
-        ERR("Couldn't create a device interface iterator: (%08x)\n", kr);
+        WARN("Couldn't create a device interface iterator: (%08x)\n", kr);
         return -1;
     }