ixgbe: add pause frame stats

Report standard pause frame stats. They are already aggregated
in struct ixgbe_hw_stats.

The combination of the registers is suggested as equivalent to
PAUSEMACCtrlFramesTransmitted / PAUSEMACCtrlFramesReceived
by the Intel 82576EB datasheet, I could not find any information
in the HW actually supported by ixgbe.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>