stmmac: dwmac-socfpga: kill init() and rename setup() to set_phy_mode()

Remove old init callback which now contains only a call to
socfpga_dwmac_setup(). Also rename socfpga_dwmac_setup() to indicate
what the function really does.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Tested-by: Marek Vasut <marex@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>