When Rx timestamping is enabled, we set the timestamp bit in SRRCTL
register for each queue, but we don't clear it when disabling. This
patch fixes igc_ptp_disable_rx_timestamp() accordingly.

Also, this patch gets rid of igc_ptp_enable_tstamp_rxqueue() and
igc_ptp_enable_tstamp_all_rxqueues() and move their logic into
igc_ptp_enable_rx_timestamp() to keep the enable and disable
helpers symmetric.
Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>