staging/comedi: Use dev_ printks in rtd520.c

fixed below checkpatch warning.
-Prefer netdev_info(netdev, ... then dev_info(dev, ... then pr_info(...  to printk(KERN_INFO ...

Signed-off-by: YAMANE Toshiaki <yamanetoshi@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>