pr_debug() should not be used in drivers

pr_debug() should not be used from drivers, add comment saying that.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 181c69c..851aa1b 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -210,6 +210,7 @@
 extern void dump_stack(void);
 
 #ifdef DEBUG
+/* If you are writing a driver, please use dev_dbg instead */
 #define pr_debug(fmt,arg...) \
 	printk(KERN_DEBUG fmt,##arg)
 #else