[RISCV] Fix obvious copy paste error.

CASE_VFMA_OPCODE_VV and CASE_VFMA_CHANGE_OPCODE_VV need to match up if we are
are to avoid "Unexpected opcode" errors, but in CASE_VFMA_CHANGE_OPCODE_VV,
CASE_VFMA_CHANGE_OPCODE_LMULS_MF2 had mistakenly been used instead of
CASE_VFMA_CHANGE_OPCODE_LMULS_MF4.