i40e: Split Flow Director descriptor config into separate function

In an effort to improve code readability I am splitting the Flow Director
filter configuration out into a separate function like we have done for the
standard xmit path.  The general idea is to provide a single block of code
that translates the flow specification into a proper Flow Director
descriptor.

Change-ID: Id355ad8030c4e6c72c57504fa09de60c976a8ffe
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>