ftgmac100: Use a scratch buffer for failed RX allocations

We can occasionally fail to allocate new RX buffers at
runtime or when starting the driver. At the moment the
latter just fails to open which is fine but the former
leaves stale DMA pointers in the ring.

Instead, use a scratch page and have all RX ring descriptors
point to it by default unless a proper buffer can be allocated.

It will help later on when re-initializing the whole ring
at runtime on link changes since there is no clean failure
path there unlike open().

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>