softfloat: Fix warn about implicit conversion from int to int8_t

Change the flag type to 'uint8_t' to fix the implicit conversion error.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Message-id: 20160810185502.32015-1-bobby.prani@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>