ixgb: Don't check for vlan group on transmit

Based on a patch from Jesse Gross.

Enable vlan tag insertion even when vlan group is not configured.

For ixgb HW both CTRL0.VME and VLE bit in the Tx descriptor need to be set
in order to enable HW acceleration.

Introduced separate functions for enabling/disabling of vlan tag stripping
similar to ixgbe.

CC: Jesse Gross <jesse@nicira.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>