[RISCV][MC] Fix encoding for vfwmaccbf16

The encoding matched the one given in the bf16 extension specification
PDF, but per https://github.com/riscv/riscv-bfloat16/issues/45 it seems
this encoding was not the one that is intended and was incorrectly
modified due to an issue in the PDF generation process. This patch
corrects the opcode to 111011 from 100011.

The correct encoding is shown in the new spec PDF
<https://github.com/riscv/riscv-bfloat16/releases/tag/20230614>.

Differential Revision: https://reviews.llvm.org/D152894