staging/comedi: Use dev_ printks in drivers/comedi_fc.c

fixed below checkpatch warning.
- WARNING: Prefer netdev_warn(netdev, ... then dev_warn(dev, ... then pr_warn(...  to printk(KERN_WARNING ...

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