ppsi: change SNMP_MAX_OFFSET_PS to 500

Threshold value used to count offset errors
(SNMP's wrsPtpClockOffsetErrCnt object).

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>

ppsi @ 81278bd1

-Subproject commit b9f3fde488e31464343af53ca74f99f57e2ba4f9
+Subproject commit 81278bd184d5fb01478dd34aa30690c6c55a0653