USB: legousbtower: refactor endpoint retrieval

Use the new endpoint helpers to lookup the required interrupt-in and
interrupt-out endpoints.

Note that the descriptors are searched in reverse order to avoid any
regressions.

Cc: Juergen Stuber <starblue@users.sourceforge.net>
Cc: legousb-devel@lists.sourceforge.net
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>