USB: fix oops on disconnect in cdc-acm

This patch fixes an oops caused when during an unplug a device's table
of endpoints is zeroed before the driver is notified. A pointer to
the endpoint must be cached.

this fixes a regression caused by commit
5186ffee
Therefore it should go into 2.6.31

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>