net: mdio: mscc-miim: Fix the mdio controller

According to the documentation the second resource is optional. But the
blamed commit ignores that and if the resource is not there it just
fails.

This patch reverts that to still allow the second resource to be
optional because other SoC have the some MDIO controller and doesn't
need to second resource.

Fixes: 672a1c39 ("net: mdio: mscc-miim: Make use of the helper function devm_platform_ioremap_resource()")
Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Cai Huoqing <caihuoqing@baidu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>