ixgbe: dcb: IEEE PFC stats and reset logic incorrect

PFC stats are only tabulated when PFC is enabled. However in IEEE
mode the ieee_pfc pfc_tc bits were not checked and the calculation
was aborted.

This results in statistics not being reported through ethtool and
possible a false Tx hang occurring when receiving pause frames.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>