target/mips: Use FloatRoundMode enum for FCR31 modes conversion

Use the FloatRoundMode enum type introduced in commit 3dede407
("softfloat: Name rounding mode enum") instead of 'unsigned int'.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201123204448.3260804-2-f4bug@amsat.org>