target/mips: Adjust style in msa_translate_init()

target/mips: Adjust style in msa_translate_init()

While the first 'off' variable assignment is unused, it helps
to better understand the code logic. Move the assignation where
it would have been used so it is easier to compare the MSA
registers based on FPU ones versus the MSA specific registers.

Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211023214803.522078-34-f4bug@amsat.org>