tehuti: Convert ethtool get_stats_count() ops to get_sset_count()

This string query operation was supposed to be replaced by the
generic get_sset_count() starting in 2007.  Convert tehuti's
implementation.

Also remove the dummy self-test name which was not used since tehuti
does not advertise any self-tests.

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