x86/platform/olpc: Don't split string literals when fixing up the DT

It was pointed out in a review, and checkpatch.pl complains about this.
Breaking it down into multiple ofw evaluations works just as well and
reads better.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>