ACPI / APEI: fix error status check condition for CPER

In Table 18-289, ACPI5.0 SPEC, the error data length in CPER
Generic Error Data Entry can be 0, which means this generic
error data entry can have only one header. So fix the check
conditon for it.

Signed-off-by: Chen Gong <gong.chen@linux.intel.com>
Reviewed-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>