sh_eth: check sh_eth_cpu_data::dual_port when dumping registers

When adding the sh_eth_cpu_data::dual_port flag I forgot to add the flag
checks to __sh_eth_get_regs(), causing the non-existing TSU registers to
be dumped by 'ethtool' on the single port Ether controllers having TSU...

Fixes: a94cf2a6 ("sh_eth: fix TSU init on SH7734/R8A7740")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>