tsnep: Rework RX buffer allocation

Refill RX queue in batches of descriptors to improve performance. Refill
is allowed to fail as long as a minimum number of descriptors is active.
Thus, a limited number of failed RX buffer allocations is now allowed
for normal operation. Previously every failed allocation resulted in a
dropped frame.

If the minimum number of active descriptors is reached, then RX buffers
are still reused and frames are dropped. This ensures that the RX queue
never runs empty and always continues to operate.

Prework for future XDP support.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>