UBIFS: not build debug messages with CONFIG_UBIFS_FS_DEBUG disabled

With
  $ grep -e UBIFS_FS_DEBUG -e DYNAMIC_DEBUG .config
  # CONFIG_UBIFS_FS_DEBUG is not set
  CONFIG_DYNAMIC_DEBUG=y

Debug messages are kept in the object files due to the
dynamic_pr_debug() macro, even if they are never going to be printed:
  $ make fs/ubifs/super.o
  $ strings fs/ubifs/super.o | grep 'compiled on'
  compiled on:         Aug 11 2011 at 12:21:38

Use plain printk to fix this.

Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>