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