igc: Fix Rx timestamp disabling

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>