ixgbe: Clear head write-back registers on VF reset

The Tx head write-back registers are not cleared during an FLR or VF reset.
As a result a configuration that had head write-back enabled can leave the
registers set after the driver is unloaded.  If the next driver loaded doesn't
use the write-back registers this can lead to a bad configuration where
head write-back is enabled, but the driver didn't request it.

To avoid this situation the PF should be resetting the Tx head write-back
registers when the VF requests a reset.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>