net: macb: Fix race condition in driver when Rx frame is dropped

Under heavy Rx load, observed that the Hw is updating the USED bit
and it is not updating the received frame status to the BD control
field. This could be lack of resources for processing the BDs at high
data rates. Driver drops the frame associated with this BD but not
clearing the USED bit. So, this is causing hang condition as Hw
expects USED bit to be cleared for this BD.

Signed-off-by: Punnaiah Choudary Kalluri <punnaia@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>