UBI: do not link debug messages when debugging is disabled

Michal Marek spotted the same issue in UBIFS and this patch fixes UBI,
see "UBIFS: not build debug messages with CONFIG_UBIFS_FS_DEBUG disabled"

When UBI debugging is disabled, we have debugging messages defined as:

if (0)
	pr_debug()

But pr_debug macro defines data structures with debugging data and makes
the linux binary larger, even though we have "if (0)".

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>
diff --git a/drivers/mtd/ubi/debug.h b/drivers/mtd/ubi/debug.h
index 65b5b76..64fbb00 100644
--- a/drivers/mtd/ubi/debug.h
+++ b/drivers/mtd/ubi/debug.h
@@ -181,7 +181,7 @@
 
 #define ubi_dbg_msg(fmt, ...) do {                                           \
 	if (0)                                                               \
-		pr_debug(fmt "\n", ##__VA_ARGS__);                           \
+		printk(KERN_DEBUG fmt "\n", ##__VA_ARGS__);                  \
 } while (0)
 
 #define dbg_msg(fmt, ...)  ubi_dbg_msg(fmt, ##__VA_ARGS__)