softfloat: Use correct type in float64_to_uint64_round_to_zero()

In float64_to_uint64_round_to_zero() a typo meant that we were
taking the uint64_t return value from float64_to_uint64() and
putting it into an int64_t variable before returning it as
uint64_t again. Use uint64_t instead of pointlessly casting it
back and forth to int64_t.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

fpu/softfloat.c

@@ -7492,7 +7492,7 @@ uint64_t float64_to_uint64_round_to_zero(float64 a, float_status *status)
 {
     signed char current_rounding_mode = status->float_rounding_mode;
     set_float_rounding_mode(float_round_to_zero, status);
-    int64_t v = float64_to_uint64(a, status);
+    uint64_t v = float64_to_uint64(a, status);
     set_float_rounding_mode(current_rounding_mode, status);
     return v;
 }