i40e/i40evf: reduce context descriptors

We don't need context descriptors for every packet, only tso
or timesync.  This fixes a bug in the driver where it would
always add a context even if all the passed in values
to the context descriptor function were 0/default values.

Change-ID: I0101d2b893380707b5c2de61aab3e16d4310e9a1
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>