[mips] Change Feature-related types from unsigned to uint64_t in MipsAsmParser. No functional changes.

Summary: Found a couple of cases where unsigned was still being used. These two should be the last ones in the (entire) Mips backend.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D5028

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217257 91177308-0d34-0410-b5e6-96231b3b80d8