igc: read before write to SRRCTL register

commit 3ce29c17 upstream.

igc_configure_rx_ring() function will be called as part of XDP program
setup. If Rx hardware timestamp is enabled prio to XDP program setup,
this timestamp enablement will be overwritten when buffer size is
written into SRRCTL register.

Thus, this commit read the register value before write to SRRCTL
register. This commit is tested by using xdp_hw_metadata bpf selftest
tool. The tool enables Rx hardware timestamp and then attach XDP program
to igc driver. It will display hardware timestamp of UDP packet with
port number 9092. Below are detail of test steps and results.

Command on DUT:
  sudo ./xdp_hw_metadata <interface name>

Command on Link Partner:
  echo -n skb | nc -u -q1 <destination IPv4 addr> 9092

Result before this patch:
  skb hwtstamp is not found!

Result after this patch:
  found skb hwtstamp = 1677800973.642836757

Optionally, read PHC to confirm the values obtained are almost the same:
Command:
  sudo ./testptp -d /dev/ptp0 -g
Result:
  clock time: 1677800973.913598978 or Fri Mar  3 07:49:33 2023

Fixes: fc9df2a0 ("igc: Enable RX via AF_XDP zero-copy")
Cc: <stable@vger.kernel.org> # 5.14+
Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>