debugfs: introduce stub for debugfs_create_size_t() when DEBUG_FS=n

Toralf Förster <toralf.foerster@gmx.de> reported a build failure in
the WiMAX stack when CONFIG_DEBUG_FS=n

http://linuxwimax.org/pipermail/wimax/2009-January/000449.html

This is due to debugfs_create_size_t() missing an stub that returns
-ENODEV when the DEBUGFS subsystem is not configured in (like the rest
of the debugfs API).

This patch adds said stub.

Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>