sh_eth: add sh_eth_cpu_data::xdfar_rw flag

The GEther-like controllers have writeable RDFAR/TDFAR, on the others
they are read-only or just absent (on R-Car). Currently we are calling
sh_eth_is_{gether|rz_fast_ether}() in order to check if these registers
can be written to, however it would be simpler to check the new 'xdfar_rw'
bitfield in the 'struct sh_eth_cpu_data'...

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