i40e: move check of full Tx ring to outside of send loop

Move the check if the HW Tx ring is full to outside the send
loop. Currently it is checked for every single descriptor that we
send. Instead, tell the send loop to only process a maximum number of
packets equal to the number of available slots in the Tx ring. This
way, we can remove the check inside the send loop to and gain some
performance.

Suggested-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>