[RISCV] Fix off by 1 typo in decodeVMaskReg. NFC

We're decoding a 1 bit field, but checked that the value was <= 2
instead of <= 1.

This isn't a functional change because the generated disassembler code
that calls this only extracts 1 bit.

Author: topperc

llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp

@@ -258,9 +258,9 @@ static DecodeStatus DecodeVRM8RegisterClass(MCInst &Inst, uint32_t RegNo,
 static DecodeStatus decodeVMaskReg(MCInst &Inst, uint32_t RegNo,
                                    uint64_t Address,
                                    const MCDisassembler *Decoder) {
-  if (RegNo > 2) {
+  if (RegNo >= 2)
     return MCDisassembler::Fail;
-  }
+
   MCRegister Reg = (RegNo == 0) ? RISCV::V0 : RISCV::NoRegister;
 
   Inst.addOperand(MCOperand::createReg(Reg));