target/mips: Fix TX79 LQ/SQ opcodes

The base register address offset is *signed*.

Cc: qemu-stable@nongnu.org
Fixes: aaaa82a ("target/mips/tx79: Introduce LQ opcode (Load Quadword)")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230914090447.12557-1-philmd@linaro.org>

target/mips/tcg/tx79.decode

@@ -24,7 +24,7 @@
 @rs             ...... rs:5  ..... ..........  ......   &r sa=0      rt=0 rd=0
 @rd             ...... ..........  rd:5  ..... ......   &r sa=0 rs=0 rt=0
 
-@ldst            ...... base:5 rt:5 offset:16           &i
+@ldst            ...... base:5 rt:5 offset:s16          &i
 
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