ixgbe: make sure FCoE DDP user buffers are really released by the HW

When invalidating the DDP context is invalidated, the HW may not be done
with the user buffer right away. In which case, we poll the FCBUFF register
to check if the buffer valid bit is cleared or not, if not, we wait for max
100us that is guaranteed by the HW.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>