USB: cdc-acm: refactor endpoint retrieval

USB: cdc-acm: refactor endpoint retrieval

Use the new endpoint helpers to lookup the required bulk-in, bulk-out
and interrupt-in endpoints for collapsed interfaces.

Note that there is already a check verifying that there are exactly
three endpoints so we'd still be bailing out if there's an unexpected
endpoint type.

Cc: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>