Fix Mips Parser error reporting

Fix Mips Parser error reporting

[mips] On error, ParseDirective should always return false to signify that the
directive was understood.

Reviewers: dsanders, vkalintiris, sdardis

Subscribers: dsanders, llvm-commits, sdardis

Differential Revision: http://reviews.llvm.org/D19929

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268630 91177308-0d34-0410-b5e6-96231b3b80d8