sh_eth: add sh_eth_cpu_data::no_xdfar flag

The commit 6ded2865 ("sh_eth: Fix RX recovery on R-Car in case of RX
ring underrun") added a check for an bad RDFAR offset in sh_eth_rx(), so
that the code could work on the R-Car Ether controllers which don't have
this register (and TDFAR), then the commit 3365711d ("sh_eth: WARN on
access to a register not implemented in a particular chip") replaced
offset 0 with 0xffff. Adding/checking the 'no_xdfar' bit field in the
'struct sh_eth_cpu_data' instead results in less object code...

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