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>

target/mips/tcg/octeon.decode

@@ -12,7 +12,7 @@
 # BBIT132    111110 ..... ..... ................
 
 %bbit_p      28:1 16:5
-BBIT         11 set:1 . 10 rs:5 ..... offset:16 p=%bbit_p
+BBIT         11 set:1 . 10 rs:5 ..... offset:s16 p=%bbit_p
 
 # Arithmetic
 # BADDU rd, rs, rt