target/mips: Fix df_extract_val() and df_extract_df() dfe lookup

target/mips: Fix df_extract_val() and df_extract_df() dfe lookup

Actually look into dfe structure data so that df_extract_val() and
df_extract_df() can return immediate and datafield other than BYTE.

Fixes: 4701d23aef ("target/mips: Convert MSA BIT instruction format to decodetree")
Signed-off-by: Ni Hui <shuizhuyuanluo@126.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220503130708.272850-2-shuizhuyuanluo@126.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>