Platform: OLPC: Avoid a warning if the EC didn't register yet

Just return EPROBE_DEFER, so that whoever attempted to use the EC call can
defer their work.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>