PEL: fix sbe ffdc packet offset calculation
PEL: fix sbe ffdc packet offset calculation

Existing SBE ffdc packet offset calculation logic is wrong
and causing parser function failure for multiple packet
use-cases. Correct logic to calculate offset based on packet
sizen in bytes.

Tested: manually verified PEL log with two ffdc packet case.

Signed-off-by: Jayanth Othayoth <d8c0d7e9f313f85e22ced997a3958fcdeb1cb606@in.ibm.com>
Change-Id: Ibc82fead1f49fdbf6510e2c704bc3561654aafed

openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging