ixgbe: Always use context 0, even for FCoE and TSO

We were spending cycles separating the FCoE and TSO contexts even though we
always overwriting the context anyway.  Instead of doing that we can just
use context 0 for all descriptors.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>