drivers: peci: xec: Handle PECI command PING properly

Handle PECI command PING properly, also get Write FCS Byte as a check.
Now it is possible to perform a PECI Ping without crashing the bus or
blocking it for subsequent PECI transactions.
It is also possible to check whether the Ping was sucessful
or not with the Write FCS Byte.

Signed-off-by: Johannes Meister <johannes.meister@kontron.com>