ACPI: APEI: Remove unneeded result variables

Return the erst_get_record_id_begin() and apei_exec_write_register()
return values directly instead of storing them in redundant local
variables.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>