ravb: fix ring memory allocation

The driver is written as if it can adapt to a low memory situation  allocating
less RX  skbs and TX aligned buffers than the respective RX/TX ring sizes.  In
reality  though  the driver  would malfunction in this case. Stop being overly
smart and just fail in such situation -- this is achieved by moving the memory
allocation from ravb_ring_format() to ravb_ring_init().

We leave dma_map_single() calls in place but make their failure non-fatal
by marking the corresponding RX descriptors  with zero data size which should
prevent DMA to an invalid addresses.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>