ixgbe: allow ixgbe_ptp_reset to maintain current hwtstamp config

Rather than clearing the hwtstamp configuration, we should use the known
configuration requested by the user and call the function which has now
been separated from the ioctl. This means that after a reset, the
timestamp mode will be maintained rather than lost. We still can't
maintain the clock value, however.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>