sh_eth: merge sh_eth_free_dma_buffer() into sh_eth_ring_free()

While the ring allocation is done by a single function, sh_eth_ring_init(),
the ring deallocation was split into two functions (almost always called
one after the other) for no good reason. Merge  sh_eth_free_dma_buffer()
into sh_eth_ring_free() which allows us  to save space not only on the
direct calls  of the former function but also on the sh_eth_ring_init()'s
simplified error path...

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