sh_eth: gather all TSU init code in one place

The  sh_eth_cpu_data::chip_reset() method  always resets using ARSTR and
this register is always located at the start of the  TSU register region.
Therefore, we can  only call  this method if we know TSU is there and thus
simplify  the probing code a  bit...

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>