USB: cdc-acm: use idr to manage minor numbers

Use the idr-interface rather than a static table to manage minor-number
allocations.

This allows us to easily switch over to fully dynamic minor allocations
when the TTY-layer can handle that.

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