sh_eth: get R8A7740 Rx descriptor word 0 shift out of #ifdef

The only R8A7740 specific #ifdef hindering  ARM multiplatform build is  left in
sh_eth_rx(): it covers the code shifting Rx buffer descriptor word 0 by 16. Get
rid of the #ifdef by adding 'shift_rd0' field to the  'struct sh_eth_cpu_data',
making the shift dependent on it, and setting it to 1 for the R8A7740 case...

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