i40e/i40evf: Update DESC_NEEDED value to reflect larger value

i40e/i40evf: Update DESC_NEEDED value to reflect larger value

When compared to ixgbe and other previous Intel drivers the i40e and i40evf
drivers actually reserve 2 additional descriptors in maybe_stop_tx for
cache line alignment. We need to update DESC_NEEDED to reflect this as
otherwise we are more likely to return TX_BUSY which will cause issues with
things like xmit_more.

Signed-off-by: Alexander Duyck <[email protected]>
Tested-by: Andrew Bowers <[email protected]>
Signed-off-by: Jeff Kirsher <[email protected]>
Signed-off-by: David S. Miller <[email protected]>