sh_eth: add sh_eth_cpu_data::soft_reset() method

sh_eth_reset() performs a software reset which is implemented in a
completely different way for the GEther-like controllers vs the other
controllers due to a different layout of EDMR (and other factors) --
it therefore makes sense to convert this function to a mandatory
sh_eth_cpu_data::soft_reset() method and thus get rid of the runtime
controller type check via sh_eth_is_{gether|rz_fast_ether}().

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