ixgbe: Add one function that handles most of context descriptor setup

There is a significant amount of shared functionality between the checksum
and TSO offload configuration that is shared in regards to how they setup
the context descriptors.  Since so much of the functionality is shared it
makes sense to move the shared functionality into a single function and
just call that function from the two context descriptor specific routines.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>