USB: cdc-acm: add support for up to 256 devices

Increase the minor range to enable support for up to 256 devices.

Some people are hitting the current 32 device limit. Hopefully 256
minors will be enough for while still.

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