sh_eth: fix TSU init on SH7734/R8A7740

It appears that the single port Ether controllers having TSU (like SH7734/
R8A7740) need the same kind of treating in sh_eth_tsu_init() as R7S72100
currently has -- they also don't have the TSU registers related e.g. to
passing the frames between ports. Add the 'sh_eth_cpu_data::dual_port'
flag and use it as a new criterion for taking a "short path" in the TSU
init sequence in order to avoid writing to the non-existent registers...

Fixes: f0e81fec ("net: sh_eth: Add support SH7734")
Fixes: 73a0d907

 ("net: sh_eth: add support R8A7740")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>