ice: add lock around Tx timestamp tracker flush

The driver didn't take the lock while flushing the Tx tracker, which
could cause a race where one thread is trying to read timestamps out
while another thread is trying to read the tracker to check the
timestamps.

Avoid this by ensuring that flushing is locked against read accesses.

Fixes: ea9b847c

 ("ice: enable transmit timestamps for E810 devices")
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>