sh_eth: split 'sh_eth_netdev_ops'

Commit 9f861341 (sh_eth: remove SH_ETH_HAS_TSU)
removes 'const' from 'sh_eth_netdev_ops'  and modifies it in case TSU registers
are present. I've originally suggested to Iwamatsu-san to split  this structure
in two instead and afterwards Dave M. suggested doing the same.
Split 'sh_eth_netdev_ops_tsu' from 'sh_eth_netdev_ops', making both 'const', and
assigning 'ndev->detdev_ops'  depending on the presence of TSU registers.

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