UBI: fix debugging messages

Patch ab50ff68 broke UBI debugging messages:
before that commit when UBI debugging was enabled, users saw few useful
debugging messages after attaching an MTD device. However, that patch turned
'dbg_msg()' into 'pr_debug()', so to enable the debugging messages users have
to enable them first via /sys/kernel/debug/dynamic_debug/control, which is
very impractical.

This commit makes 'dbg_msg()' to use 'printk()' instead of 'pr_debug()', just
as it was before the breakage.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: stable@kernel.org [3.0+]