dev_printk(): constify the `dev' argument

Add const markings to dev_name and dev_driver_string to make it clear that
dev_printk doesn't modify dev.  This is a prerequisite to adding more
const markings to other functions make it clearer, which functions can
modify dev and which can't.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>