To allow userland to enable or disable dropping packets when descriptor
ring is exhausted, add an adapter rx_drop_en attribute.

This can be used in conjunction with flow control to mitigate packet storms
(e.g. due to network loop or DoS) by forcing the network adapter to send
pause frames whenever the ring is close to exhaustion.

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>