net: ethoc: implement ethtool get/set ring parameters

TX and RX rings share memory and descriptors. Maximal number of
descriptors reported is one less than the total available nuber of
descriptors. For the set operation the requested number of TX descriptors
is rounded down to the nearest power of two (driver logic requirement).

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>