[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.

GitOrigin-RevId: 5dd46d93fb9d7e7f8c9774433d60fd16dc659eb6
1 file changed