net/macb: improved ethtool statistics support

Currently `ethtool -S` simply returns "no stats available". It
would be more useful to see what the various ethtool statistics
registers' values are. This change implements get_ethtool_stats,
get_strings, and get_sset_count functions to accomplish this.

Read all GEM statistics registers and sum them into
macb.ethtool_stats. Add the necessary infrastructure to make this
accessible via `ethtool -S`.

Update gem_update_stats to utilize ethtool_stats.

Signed-off-by: Xander Huff <xander.huff@ni.com>
Signed-off-by: David S. Miller <davem@davemloft.net>