net: Convert ethtool {get_stats, self_test}_count() ops to get_sset_count()

These string query operations were supposed to be replaced by the
generic get_sset_count() starting in 2007.  Convert the remaining
implementations.

Also remove calls to these operations to initialise drvinfo->n_stats.
The ethtool core code already does that.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>