mv643xx_eth: Hook up mv643xx_get_sset_count

Commit b9f2c044 replaced mv643xx_get_stats_count() with
mv643xx_get_sset_count(), but forgot to hook it up.

drivers/net/mv643xx_eth.c:2678: warning: mv643xx_get_sset_count defined but not used

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>