USB: correct error handling in cdc-wdm

This patch to cdc-wdm

- checks for partial extra descriptors
- fixes a leak in the error case of probe
- checks for an exact number of endpoints
- adds a clarifying comment

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>