nfp: prepare for multi-part descriptors

New datapaths may use multiple descriptor units to describe
a single packet.  Prepare for that by adding a descriptors
per simple frame constant into ring size calculations.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Fei Qin <fei.qin@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>