ACPI, APEI, Fix ERST header length check

This fixes a trivial copy & paste error in ERST header length check.
It's just for future safety because sizeof(struct acpi_table_einj)
equals to sizeof(struct acpi_table_erst) with current ACPI5.0
specification. It applies to v3.3-rc6.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Acked-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>