MIPS: Fix of r16681

Use v0 for return value.

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/23437032

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16694 ce2b1a6d-e550-0410-aec6-3dcde31c8c00