target/mips: Clean up dsp_helper.c

Remove several minor checkpatch warnings and errors.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <1556018982-3715-6-git-send-email-aleksandar.markovic@rt-rk.com>