net: eth: cpsw: Correctly attach to GPIO bitbang MDIO driver

When the GPIO bitbang MDIO driver is used instead of the Davinci MDIO driver
we need to configure the phy_id string differently. Otherwise this string
looks like this "gpio.6" instead of "gpio-0" and the PHY is not found when
phy_connect() is called.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Lukas Stockmann <lukas.stockmann@siemens.com>
Cc: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>