sh_eth: advance 'rxdesc' later in sh_eth_ring_format()

Iff dma_map_single() fails, 'rxdesc'  should point  to the last filled RX
descriptor, so  that it can be marked as the last one, however the driver
would have  already  advanced it by that time. In order to fix that, only
fill  an RX descriptor  once all the data for it is ready.

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