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: NJohan Hovold <johan@kernel.org>
Acked-by: NOliver Neukum <oneukum@suse.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>