mtd: ubi, ubifs debug: Use pr_debug instead of pr_crit

Before printk.h was introduced and MTDDEBUG was removed,
pr_crit() was calling MTDDEBUG(), which was since then
replaced by the current pr_debug().

pr_debug is more appropriate here.

Signed-off-by: Eran Matityahu <eran.m@variscite.com>
Reviewed-by: Heiko Schocher <hs@denx.de>