target/mips: Cast offset field of Octeon BBIT to int16_t

target/mips: Cast offset field of Octeon BBIT to int16_t

As per "Cavium Networks OCTEON Plus CN50XX Hardware Reference
Manual" offset field is signed 16 bit value. However arg_BBIT.offset
is unsigned. We need to cast it as signed to do address calculation.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221031132531.18122-3-jiaxun.yang@flygoat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>