target/riscv: Fix vfwmaccbf16.vf

target/riscv: Fix vfwmaccbf16.vf

The operator (fwmacc16) of vfwmaccbf16.vf helper function should be
replaced by fwmaccbf16.

Fixes: adf772b0f7 ("target/riscv: Add support for Zvfbfwma extension")
Signed-off-by: Max Chou <max.chou@sifive.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231005095734.567575-1-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
(cherry picked from commit 837570cef237b634eb4c245363470deebea7089d)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>