Read stale PTP Tx timestamps from PHY on cleanup.

After running out of Tx timestamps request handlers, hardware (HW) stops
reporting finished requests. Function ice_ptp_tx_tstamp_cleanup() used
to only clean up stale handlers in driver and was leaving the hardware
registers not read. Not reading stale PTP Tx timestamps prevents next
interrupts from arriving and makes timestamping unusable.

Fixes: ea9b847c ("ice: enable transmit timestamps for E810 devices")
Signed-off-by: Michal Michalik <michal.michalik@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>