MFC r277136:
Resolve a special case deadlock: When two or more threads are
simultaneously detaching kernel drivers on the same USB device we can
get stuck in the "usb_wait_pending_ref_locked()" function because the
conditions needed for allowing detach are not met.

While at it ensure that "flag_iserror" is only written when "priv_mtx"
is locked, which is protecting it.