sh_eth: make sh_eth_tsu_get_offset() match its name

sh_eth_tsu_get_offset(), despite its name, returns a TSU register's address,
not its offset.  Make this  function match its name and return a register's
offset  from the TSU  registers base address instead.

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