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>

drivers/staging/line6/driver.h

@@ -53,7 +53,7 @@
 #define LINE6_CHANNEL_MASK 0x0f
 
 #define MISSING_CASE	\
-	printk(KERN_ERR "line6usb driver bug: missing case in %s:%d\n", \
+	pr_err("line6usb driver bug: missing case in %s:%d\n", \
 		__FILE__, __LINE__)
 
 #define CHECK_RETURN(x)		\