[AArch64] validateTargetOperandClass - early out from MCK_MPR case. NFCI.

If it didn't match a za register, there's nothing we can do.

Fixes static analyzer uninitialized variable warning.

llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp

@@ -6883,7 +6883,7 @@ unsigned AArch64AsmParser::validateTargetOperandClass(MCParsedAsmOperand &AsmOp,
     // as a literal token.
     if (Op.isTokenEqual("za"))
       return Match_Success;
-    break;
+    return Match_InvalidOperand;
   }
   if (!Op.isImm())
     return Match_InvalidOperand;