RISC-V: Fix T-Head Fmv vendor extension encoding

A recent change in the XTheadFmv spec fixed an encoding bug in the
document. This patch changes the code to follow this bugfix.

Spec patch can be found here:
  https://github.com/T-head-Semi/thead-extension-spec/pull/11

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>