kernel: backport mdio_find_bus from 5.10.x

kernel: backport mdio_find_bus from 5.10.x

mdio-tools (https://github.com/wkz/mdio-tools) requires the use of
mdio_find_bus, which is not present in 5.4.x.

This patch backports the required change from 5.10.x to 5.4.x

The original patch submission for this can be found at
https://patchwork.ozlabs.org/project/netdev/patch/20200201074625.8698-2-jeremy.linton@arm.com/

Signed-off-by: Damien Mascord <tusker@tusker.org>
[Add original patch description]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>