x86, olpc: Don't retry EC commands forever

Avoids a potential infinite loop.

It was observed once, during an EC hacking/debugging
session - not in regular operation.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Cc: dilinger@queued.net
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>