[MTD] [NOR] gen_probe: No debug message when debugging is disabled

Use pr_debug(...) instead of printk(KERN_DEBUG ...) so that the message
is only printed when debugging is enabled.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Tested-by: John stoffel <john@stoffel.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>