streamline desc buffer processing

Steal a page from rocker driver and use a struct desc_info to track desc and
desc buf.  Uses fixed backing memory for descs and bufs, allocate once on first
use.  Avoids reallocation of desc buf backing memory each time a desc is used.

Tested with current rocker driver and ethtool eth1.

Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
simplify front-panel port initialization

Configure, create nic, and create netdev backend in one shot during
initialization of front-panel port.  No need for individual funcs for each
step, just do it all in front-panel alloc.

Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>