sh_eth: use TSU register accessors for TSU_POST<n>

There's no particularly good reason TSU_POST<n> registers get accessed
circumventing sh_eth_tsu_{read|write}() -- start using those, removing
(badly named) sh_eth_tsu_get_post_reg_offset(),  while at it...

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