smsc95xx: enable dynamic autosuspend

This patch enables USB dynamic autosuspend for LAN9500A.  This
saves very little power in itself, but it allows power saving
in upstream hubs/hosts.

The earlier devices in this family (LAN9500/9512/9514) do not
support this feature.

Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>