Various hardware statistics that are available for Siena are
unavailable or meaningless for Falcon.  Huntington adds further to the
NIC-type-specific statistics, as it has different MAC blocks from
Falcon/Siena.

All NIC types still provide most statistics by DMA, and use
little-endian byte order.

Therefore:
1. Add some general utility functions for reporting hardware statistics,
   efx_nic_describe_stats() and efx_nic_update_stats().
2. Add an efx_nic_type::describe_stats operation to get the number and
   names of statistics, implemented using efx_nic_describe_stats()
3. Change efx_nic_type::update_stats to store the core statistics
   (struct rtnl_link_stats64) or full statistics (array of u64) in a
   caller-provided buffer.  Use efx_nic_update_stats() to aid in the
   implementation.
4. Rename struct efx_ethtool_stat to struct efx_sw_stat_desc and
   EFX_ETHTOOL_NUM_STATS to EFX_ETHTOOL_SW_STAT_COUNT.
5. Remove efx_nic::mac_stats and struct efx_mac_stats.
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>