i40e: Collect PFC XOFF RX stats even in single TC case

When PFC is enabled for any UP in single TC configuration the driver didn't
collect the PFC XOFF RX stats. Though a single TC with PFC enabled is not a
common scenario do not prevent the driver from collecting stats if firmware
indicates that PFC is enabled.

Change-ID: Ie20bd58b07608b528f3c6d95894c9ae56b00077a
Signed-off-by: Neerav Parikh <neerav.parikh@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>