ACPI / APEI: Fix the returned value in erst_dbg_read

If the persistent store is empty initially, the function 'erst_dbg_read'
returns a nonzero value. The better way is to return a zero indicating the
read operation reaches EOF.

Tested on two different servers.

Signed-off-by: Adrian Huang <adrian.huang@hp.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>