ixgbe: Fix checking TFCS register for TXOFF status when DCB is enabled

When DCB is enabled, the ixgbe_check_tx_hang() should check the corresponding
TC's TXOFF in TFCS based on the TC that the tx ring belongs to. Adds a
function to map from the tx_ring hw reg_idx to the correspodning TC and read
TFCS accordingly.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>