enic: improve Rx performance

enic: improve Rx performance

This is a wholesale replacement of the Enic PMD receive path in order
to improve performance and code clarity. The changes are:
- Simplify and reduce code path length of receive function.
- Put most of the fast-path receive functions in one file.
- Reduce the number of posted_index updates (pay attention to
  rx_free_thresh)
- Remove the unneeded container structure around the RQ mbuf ring
- Prefetch next Mbuf and descriptors while processing the current one
- Use a lookup table for converting CQ flags to mbuf flags.

Signed-off-by: John Daley <johndale@cisco.com>