hw/sd/ssi-sd: fix error handling in ssi_sd_realize

hw/sd/ssi-sd: fix error handling in ssi_sd_realize

It's wrong to use same err object as errp parameter for several
function calls without intermediate checking for error: we'll crash if
try to set err object twice. Fix that.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200317125741.15301-1-vsementsov@virtuozzo.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>