sh_eth: get Ether port # only when needed

The dual-port Ether configurations always have a shared TSU to e.g. pass
the packets between those  ports.  With the  TSU init. code gathered under
the single *if*, we now can only get the port # from 'platform_device::id'
only when we actually  need it  (and not recalculate it each time)...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>