USB: usblp: refactor endpoint retrieval

Use the new endpoint helpers to lookup the required bulk-out endpoint
and the depending on protocol likewise required bulk-in endpoint.

Cc: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>