ixgbe: Update DESC_NEEDED define to adjust for changes to MAX_SKB_FRAGS

ixgbe: Update DESC_NEEDED define to adjust for changes to MAX_SKB_FRAGS

Recent changes have made it so that MAX_SKB_FRAGS is now never less than 16.
As a result we were seeing issues on systems with 64K pages as it would
cause DESC_NEEDED to increase to 68, and we would need over 136 descriptors
free before clean_tx_irq would wake the queue.

This patch makes it so that DESC_NEEDED is always MAX_SKB_FRAGS + 4.  This
should prevent any possible deadlocks on the systems with 64K pages as we will
now only require 42 descriptors to wake.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>