mpc83xx: gazerbeam: Update sysinfo_get error handling

In a later patch sysinfo_get will be changed to return the device in cae
of an error. Set sysinfo to NULL on error to preserve previous behavior.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>