x86, olpc: Constify an olpc_ofw() arg

The arguments passed to OFW shouldn't be modified; update the 'args'
argument of olpc_ofw to reflect this.  This saves us some later
casting away of consts.

Signed-off-by: Andres Salomon <dilinger@queued.net>
LKML-Reference: <20100628220029.1555ac24@debian>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>