i40e/i40evf: Break up xmit_descriptor_count from maybe_stop_tx

In an upcoming patch I would like to have access to the descriptor count
used for the data portion of the frame.  For this reason I am splitting up
the descriptor count function from the function that stops the ring.

Also in order to try and reduce unnecessary duplication of code I am moving
the slow-path portions of the code out of being inline calls so that we can
just jump to them and process them instead of having to build them into
each function that calls them.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>