USB: fix USB boot crash, ecm_do_notify(), list_add corruption. prev->next should...

This fixes a BUG() turned up by Ingo via randconfig testing, where
CONFIG_LIST_DEBUG turned up list corruption.  The corruption was
caused by the dummy_hcd (single-machine test harness for gadget and
HCD code) trashing the request queue when driven by the new CDC
composite gadget an I/O pattern that was previously uncommon.
Fix suggested by Alan Stern.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>