staging: line6: use pr_err() instead of printk(KERN_ERR, ...)

Fix the following checkpatch.pl warning:

  WARNING: Prefer netdev_err(netdev, ... then dev_err(dev, ... then pr_err(...  to printk(KERN_ERR ...
  #1861: FILE: staging/line6/driver.h:56:
  +       printk(KERN_ERR "line6usb driver bug: missing case in %s:%d\n", \

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>