[M68k] Replace OperandMatchResultTy with ParseStatus (NFC)

ParseStatus is slightly more convenient to use due to implicit
conversion from bool, which allows to do something like:
```
  return Error(L, "msg");
```
when with MatchOperandResultTy it had to be:
```
  Error(L, "msg");
  return MatchOperand_ParseFail;
```
It also has more appropriate name since parse* methods are not only for
parsing operands.

Reviewed By: myhsu

Differential Revision: https://reviews.llvm.org/D154320