fpu: Fix a comment in softfloat-types.h

fpu: Fix a comment in softfloat-types.h

In softfloat-types.h a comment documents that if the float_status
field flush_to_zero is set then we flush denormalised results to 0
and set the inexact flag.  This isn't correct: the status flag that
we set when flush_to_zero causes us to flush an output to zero is
float_flag_output_denormal_flushed.

Correct the comment.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250124162836.2332150-22-peter.maydell@linaro.org