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>

drivers/mtd/ubi/debug.h

@@ -181,7 +181,7 @@ static inline int ubi_dbg_is_erase_failure(const struct ubi_device *ubi)
 
 #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__)