USB: remove unneeded WARN_ON

This patch (as883) removes an out-of-date WARN_ON from the main HCD
endpoint-disable routine.  The warning is triggered whenever an
endpoint is disabled while the root hub is suspended.  In the past
that may not have been legal, but it definitely is legal now.  Merely
unbinding a USB driver will do it.

Furthermore, I've never seen any occurrences of this warning that
really did signal an actual bug or error condition.  At this point it
has outlived its purpose.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>