sh_eth: uninline sh_eth_soft_swap()

sh_eth_tsu_soft_swap() is called twice by the driver, remove *inline* and
move  that function  from the header to the driver itself to let gcc decide
whether to expand it inline or not...

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>