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>