net: mdio-sun4i: Convert to devm_* api

Use devm_ioremap_resource instead of of_iomap() and devm_kzalloc()
instead of kmalloc() to make cleanup paths simpler. This patch also
fixes the resource leak caused by missing corresponding iounamp()
of the of_iomap().

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>