sh_eth: factor out common code from MDIO bitbang methods

sh_mm[cd]_ctrl()  and sh_set_mdio() all look mostly the same -- factor out
their common code and put it into sh_mdio_ctrl().

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>