net: socionext: Increase descriptors to 256

Increasing descriptors to 256 from 128 and adjusting the NAPI weight
to 64 increases performace on Rx by ~20% on 64byte packets

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>