ixgbe: fix bug with vlan strip in promsic mode

The ixgbe driver was setting up 82598 hardware correctly, so that
when promiscuous mode was enabled hardware stripping was turned
off.  But on 82599 the logic to disable/enable hardware stripping
is different, and the code was not updated correctly when the
hardware vlan stripping was enabled as default.

This change comprises the creation of two new helper functions
and calling them from the right locations to disable and enable
hardware stripping of vlan tags at appropriate times.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>