igb: Consolidate creation of Tx context descriptors into a single function

This patch is meant to simplify the transmit path by reducing the overhead
for creating a transmit context descriptor.  The current implementation is
split with igb_tso and igb_tx_csum doing two separate implementations on
how to setup the tx_buffer_info structure and the tx_desc.  By combining
them it is possible to reduce code and simplify things since now only one
function will create context descriptors.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>