ixgbevf: Consolidate Tx context descriptor creation code

There is a good bit of redundancy between the Tx checksum and segmentation
offloads.  In order to reduce some of this I am moving the code for
creating a context descriptor into a separate function.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>