sh_eth: call sh_eth_tsu_get_offset() from TSU register accessors

With sh_eth_tsu_get_offset() now actually returning TSU register's offset,
we  can at last use it in sh_eth_tsu_{read|write}(). Somehow this saves 248
bytes of object code with AArch64 gcc 4.8.5... :-)

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