sh_eth: add sh_eth_cpu_data::edtrr_trns value

sh_eth_get_edtrr_trns() returns the value to be written to EDTRR in order
to start TX DMA -- this value is different between the GEther-like and
the other controllers. We can replace this function (and thus get rid of
the calls to sh_eth_is_{gether|rz_fast_ether}() by it) with a new field
'edtrr_trns' 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>