net: ethernet: ave: enable Rx drop interrupt

This enables AVE_GI_RXDROP interrupt factor. This factor indicates
depletion of Rx descriptors and the handler counts the number
of dropped packets.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: David S. Miller <davem@davemloft.net>