ice: re-arrange some static functions in ice_ptp.c

A following change is going to want to make use of ice_ptp_flush_tx_tracker
earlier in the ice_ptp.c file. To make this work, move the Tx timestamp
tracking functions higher up in the file, and pull the
ice_ptp_update_cached_timestamp function below them. This should have no
functional change.
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>