hw/ssi: Update coding style to make checkpatch.pl happy

To make the next commit easier to review, clean this code first.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201012124955.3409127-3-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>