Platform: OLPC: Fix olpc_xo175_ec_cmd() return value

Reset the ret variable to make sure it olpc_xo175_ec_cmd() ends up
returning zero on success.

Fixes: 0c3d931b ("Platform: OLPC: Add XO-1.75 EC driver")
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>